Human Eye And Colorful World - Class 10 Physics - Extra Questions
The impression of an image on the brain lasting for one-sixteenth of a second is called ........
Answer the following.State the function of iris and ciliary muscles in human eye.
Draw a labeled sketch of the human eye.
The figure shows a labelled sketch of the human eye.
Why can't we see clearly through fog? Name the phenomenon responsible for it.
Why is a normal eye not able to see clearly the objects placed closer than 25 cm?
What is the ability of the eye lens to adjust its focal length called?
Which part of the human eye helps in changing the focal length of the eye lens?
A doctor has prescribed a corrective lens of power $$+1.5D$$. Find the focal length of the lens. Is the prescribed lens diverging or converging?
Myopia can be corrected by using _____ lens.
What does this $$\lambda$$ symbol indicates in the context of sound wave?
The symbol $$\lambda$$ indicates the wavelength of the sound wave.
An object is moved closer to an eye. What changes must take place in the eye in order to keep the image in sharp focus?
A swimmer sees only hazy contours of objects when he opens his eyes under water, while they are distinct when using a mask. Why?
When light rays fall on the cornea of the eye, they refract if the medium outside the cornea is air. But if the medium outside the cornea is water then no significant refraction takes place as the refractive index of water is quite similar to that of the aqueous humor or vitreous humor. If there is no refraction, then the light rays will not converge on the retina to form an image.
When the swimmer wears a mask, the medium outside cornea is air and hence the refraction takes place. So the light rays converge at the retina after refraction and therefore a clear image is formed.
Draw ray diagrams each showing (i) myopic eye and (ii) hypermetropic eye.
i) Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has the far point nearer than infinity. Such a person may see clearly up to a distance of a few meters. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself.
ii) Hypermetropia is also known as farsightedness. A person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly. The near point, for the person, is farther away from the normal near point $$(25\ cm)$$.
Write down the names of parts of the eye in the blank spaces shown in Fig. 16.10.
A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her? Draw ray diagram for the correction of this defect.
If distant objects appear to be blurred or not clearly visible then the student has near-sighted, or myopia. In this case a concave lens is used to correct the problem.
Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect may arise due to (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball. This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
A person is advised to wear spectacles with convex lenses. State the defect of vision he is suffering from.
Since the convex lens, a diverging lens is recommended hence the person is suffering from hypermetropia. The power of the convex lens adds up with the power of the eye lens to focus the image at the retina.
What happens to the image distance from the eye lens in the normal eye when we increase the distance of an object from the eye ?
When an object can be seen with one eye, why is it that we have a pair of eyes?
There are several advantages of us having two eyes instead of one.
- Two eyes give us a wider field of view. A human being has a horizontal field of view of about $$150^o$$ with one eye and of about $$180^o$$ with two eyes.
- The ability to detect faint objects is also enhanced with two detectors instead of one.
- The two eyes give us a sense of depth. It makes us see the world in 3-D. With one eye, we wouldn't be able to sense or detect the depth or distance of objects in front of us.
- The parallax error is also reduced when things are viewed through 2 eyes.
Thus, we have a pair of eyes than one single eye.
An eye is said to be long-sighted if the image of a distant object is formed in front of the retina.
What is a long-sighted eye ? State one cause of long-sightedness. Name the lens used for correcting long-sightedness.
What is meant by the power of accommodation of eye?
Given is a diagram showing a defect of human eye.
Study it and answer the following question :Give two possible reasons for this defect of eye in human being.
Give reasons: A concave lens is used to correct myopia.
In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. A concave(diverging) lens causes light rays to diverge before they strike the lens of the eye so that the image is formed on the retina. Hence, a concave lens is used to correct myopia.
Give reasons:The sun appears reddish early in the morning.
Fill in the blanks:Very fine particles mainly scatter ___________ light.
Give reasons:Stars twinkle at night.
Distinguish between Myopia and Hypermetropia.
Give reasons. In old age bifocal lens is necessary for person.
Where else in nature is this phenomenon observed ?
The formation of rainbow is caused by the dispersion of the white sunlight into its constituent colours. The phenomenon of the splitting up of the white light into its constituent colours is called dispersion of light. Dispersion of light is caused because different constituent colours
of light offer different refractive indexes to the material of the prism.
of light offer different refractive indexes to the material of the prism.
Tarun wears spectacles having convex lenses. When he tries to read a book by keeping it at a distance of 25 cm, the text print seems to be blurred. What conclusion can be drawn about the least distance of distinct vision of Tarun's eye ?
How does the defect of myopia arise in a human eye?
Draw a diagram to show how hypermetropia is corrected.
Hypermetropia is corrected using a convex lens of appropriate power. The convex lens converges the light rays so that the final image from human eye lens is formed on the retina.
Explain myopia with the help of a ray diagram.
Myopia or short sightedness is the defect in which person is unable to focus far off objects clearly. This happens as the eye is unable to relax its eye lens enough to form the image on retina and the final image is formed in front of retina. This causes the person to see blurred images of far off objects.
A person is looking at another person wearing a shirt with a pattern comprising of vertical and horizontal lines. He is able to see the vertical lines more distinctly than the horizontal ones. What is this defect due to? How is such a defect of vision corrected?
Does short-sightedness (myopia) or long-sightedness (hypermetropia) imply necessarily that the eye has partially lost its ability of accommodation? If not, what might cause these defects of vision?
A myopic person has been using spectacles of power $$-1.0\ D$$ for distant vision. During old age, he also needs to use a separate reading glass of power $$+ 2.0\ D$$. Explain what may have happened.
The far point of a myopic person is $$80\ cm$$ in front of the eye. What is the nature and power of lens required to correct the problem?
What happens to the image distance in the eye when we increase the distance of an object from the eye?
Why does the Sun appear reddish early in the morning?
Make a diagram to show how hypermetropia is corrected. The near point of hypermetropic eye is 1 m. What is the power of lens required to correct this defect? Assume that the near point of the normal eye is 25 cm.
Hypermetropia can be corrected by using a convex lens. A convex lens converges the incoming light such that the image is formed on the retina.
An object at 25 cm forms an image at the near point of the hypermetropic eye. Here, near point is 1 m.
Given,
Object distance,$$u=-25\ cm$$
Image distance, $$v=-100\ cm$$
From lens formula, $$\cfrac{1}{v}-\cfrac{1}{u}=\cfrac{1}{f}$$
$$\cfrac{1}{-100}-\cfrac{1}{-25}=\cfrac{1}{f}$$
Focal length,$$f=100/3\ cm = 1/3\ m$$
Power, $$P=\cfrac{1}{f}=\cfrac{1}{1/3}=3\ D$$
A person needs a lens of power -5.5 diopters for correcting his distant vision. For correcting his near vision he needs a lens of power +1.5 diopters. What is the focal length of the lens required for correcting (i) distant vision (ii) near vision?
Why does the sky appear dark instead of blue to an astronaut?
What is atmospheric refraction? Use this phenomenon to explain the advanced sun-rise and delayed sun-set.
When light enters from one medium to another,
it deviates from its path and this phenomenon is called refraction of light.
Atmosphere is composed of layers of optical densities and due to the variation
in air density there is the deviation of light or other electromagnetic wave
from a straight line when it passes through the atmosphere known as
Atmospheric refraction.
Because of Atmospheric refraction, the light rays travel a curved path as shown in the figure. During sunrise, light rays bend and reach eyes earlier than actual line of sun and during sunset, light rays bend and the sun is visible for a longer time than that if the sun-rays were straight lines.
Hence, the Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset. Actual sunrise, mean the actual crossing of the horizon by the Sun. The time difference between actual sunset and the apparent sunset is approx. 2 minutes.
The apparent flattening of the Sun's disc at sunrise and sunset is also because of Atmospheric refraction.
(a) List three common refractive defects of vision. Suggest a way of correcting these defects.
(b) About $$45\ lac$$ people in developing countries are suffering from corneal blindness. About $$30\ lac$$ children below the age of $$12$$ years suffering from this defect can be cured by replacing the defective cornea with the cornea of a donated eye. How and why can students of your age involve themselves to create awareness about this fact among people?
Draw a labelled ray diagram to illustrate the dispersion of a narrow beam of white light when it passes through a glass prism.
Why does bluish colour predominate in a clear sky?
What is meant by scattering of light? Use this phenomenon to explain why the clear sky appears blue or the Sun appears reddish at sunrise.
Write the importance of ciliary muscles in the human eye. Name the defect of vision that arises due to gradual weakening of the ciliary muscles. What types of lenses are required by the person suffering from this defect to see the objects clearly?
Akshay, sitting in the last row in his class, could not see clearly the words written on the blackboard. When the teacher noticed it, he announced if any student sitting in the front row could volunteer to exchange his seat with Akshay. Salman immediately agreed to exchange his seat with Akshay. He could now see the words written on the blackboard clearly. The teacher thought it fit to send the message to Akshay's parents advising them to get his eyesight checked.
In the context of the above event, answer the following questions:
(a) Which defect of vision is Akshay suffering from? Which type of lens is used to correct this defect?
An old man cannot see objects closer than 1 m from the eye clearly. Name the defect of vision he is suffering from. How can it be corrected? Draw ray diagram for the (i) defect of vision and also (ii) for its correction.
The man is suffering from hypermetropia, or farsightedness. He cannot see nearby objects clearly because the eyelens does not have enough converging power to converge the light from nearby objects on the retina. Hence an external converging lens is used to focus on the nearby objects.
(a) A person cannot read newspaper placed nearer than 50 cm from his eyes. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect. List its two possible causes. Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.
(b) We see advertisements for eye donation on television or in newspapers. Write the importance of such advertisements.
A man cannot see objects closer than 1 meter from the eye clearly. What is the power of the corrective lens he should use?
Calculate the power of the eye - lens of the normal eye when it is focused at its (a) far point, infinity and (b) near point, 25 cm from the eye. Assume the distance of the retina from the eye - lens to be 2.5 cm.
The defect hypermetropia of an eye can be corrected by using spectacles containing ....................... lens.
Explain, why white light is dispersed when passing through a prism.
On what factor does refractive index of a prism depend?
Explain the correction of the eye defect Hypermetropia.
$$(i)$$ Why does the Sun appear red at sunrise?
$$(ii)$$ Name the subjective property of light related to its wavelength.
How do you correct the eye defect Myopia?
What do you understand by the term 'Scattering of light'? Which colour of white light is scattered the least and why?
A person suffering from myopia cannot see beyond 1 m. What should be the focal length of the concave lens that will correct his vision?
A man cannot see objects closer than 1 metre from the eye clearly. What is the power of the corrective lens he should use?
Explain two activities for the formation of artificial rainbow.
Activity - I:
- Fill a glass with water
- Place the glass on a table in front of a sunny window.
- Place a sheet of paper on the floor. It could be any light color. White might be best, but try any color you prefer.
- Wet the window with hot water.
- Adjust the glass and the paper until you see a rainbow
- Take a tray and fill with water.
- Place a mirror in water such that it makes an angle with the water surface.
- Now focus white light on the mirror through the water.
- Keep a white cardboard sheet above the water surface.
- We may observe the colors VIBGYOR on the board.
Explain the formation of rainbow.
Fill in the blanks
Myopia can be corrected by using __________ lens.
Myopia can be corrected by using concave lens.
Hypermetropia can be corrected by using ____________ lens.
How do you appreciate the working of Ciliary muscles in the eye?
Fill in the blanks
The eye lens can change its focal length due to working of _________ muscles.
The focal length of a lens suggested to a person with Hypermetropia is $$100 cm$$. Find the distance of near point and power of the lens.
The distance between the eye lens and retina is about __________.
Eye is the only organ to visualise the colourful world around us. This is possible due to accommodation of eye lens. Prepare a six line stanza expressing your wonderful feelings.
Fill in the blanks
The maximum focal length of the eye lens is about _________.
In the atmosphere which colour of light gets scattered the least?
The image shows an eye defect. Which defect is it?
The figure given in the question is showing a hypermetropia defect of the human eye, which is also called farsightedness. The person suffering from this defect is unable to see the nearby objects clearly as the image is formed behind the retina.
The defect can be corrected using a convex lens.
Rewrite the following table so as to match the second and third column with the first column :
| Column I | Column II | Column III | |
| 1 | Myopia | Old age problem | Bifocal lens |
| 2 | Presbyopia | Near-sightedness | Concave lens |
Given diagram is showing a defect of a human eye. Study it and answer the following question:Give the reason for this defects of the eye in the human being.
What is myopia? Explain two possible reasons of myopia. How can it be corrected? Explain with suitable diagram.
Myopia or short sightedness is a defect which arises due to slight elongation of the eyeball or due to bulging of the eye lens. This increases the distance between the eye lens and retina.In this person can not see clearly distant objects.
This defect can be corrected by using concave lens as shown in the figure, it will diverge the light ray such that after refraction from the eye lens it converges on the retina
Rewrite the following table so as to match the second and third columns with the first column :
| Column I | Column II | Column III |
| (1) Dispersion | Long - Sightedness | Twinkling of Stars |
| (2) Refraction | Splitting of white light into component colors. | Convex lens |
| (3) Hypermetropia | Change in the direction of ray of light due to change in medium. | Spectrum of seven colours |
Given below is a diagram showing a defect of the human eye. Study it and answer the following question:Name the type of lens used to correct the eye defect.
Given below is a diagram showing a defect of the human eye. Study it and answer the following question:
Name the type of lens used to correct the eye defect.
According to the images given in the question, Image of an object is forming behind the retina. Hence this defect is known as Far-sightedness or Hypermetropia.
For its correction, converging lens are used with suitable power.
What is dispersion of white light? What is the cause of such dispersion? Draw a diagram to show the dispersion of white light by a glass prism.
The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light.
Cause of dispersion of light:
When light passes through a prism, the different frequencies of light travel at different velocities. Because of refraction, the different velocities make the angles of refraction different, causing the light to travel in slightly different directions.
The diagram demonstrating the dispersion of white light by a glass prism is shown.
My grandfather uses a bifocal lens in his spectacle. Explain why?
Name the two major defects of an eye. Explain causes of their development and discuss methods of their correction.
The sky appears dark instead of blue to an astronaut.Why?
What is meant by short-sightedness?
Choose the correct answer from the alternatives given in the brackets.
The defect myopia can be corrected by using a ..............
(Convex lens, Concave lens, Concave mirror, Convex mirror)
What type of lens is used to correct long-sightedness?
Long sightedness is a defect of vision in which a person can see distant objects clearly but nearby objects appear blurred. In this defect, the light rays focus behind the retina.
Convex lens is used to correct long sightedness.
Why is hypermetropia caused? How can it be corrected?
Hypermetropia arises because of either the focal length of the eye lens
is too large or the eyeball becomes too short, so that light rays from
the nearby object, cannot be brought to focus on the retina to give a
distinct image.
This defect can be corrected by using a convex (converging) lens of appropriate focal length.
This defect can be corrected by using a convex (converging) lens of appropriate focal length.
Explain the formation of a mirage.
From the following description write down the names of the parts in human eye.
(a) Dark muscular diaphragm that controls the pupil.
(b) The screen where the image is formed by eye lens.
Explain briefly the reason for the blue colour of the sky.
What is Hypermetropia? How can it be corrected?
Observe the diagram and fill up the following:
a. The diagram shows .......... defect of the eye.
b. .......... lens is used to correct this defect.
What is myopia? How can it be corrected?
Nearsightedness or myopia is the inability of the eye to focus on distant objects. The nearsighted eye has no difficulty viewing nearby objects. But the ability to view distant objects requires that the light be refracted less. Nearsightedness will result if the light from distant objects is refracted more than is necessary. If the cornea bulges more than its customary curvature, then it tends to refract light more than usual. This tends to cause the images of distant objects to form at locations in front of the retina. If the eyeball is elongated in the horizontal direction, then the retina is placed at a further distance from the cornea-lens system. Subsequently the images of distant objects form in front of the retina. On the retinal surface, where the light-detecting nerve cells are located, the image is not focused. These nerve cells thus detect a blurry image of distant objects.
Correction-The cure for the nearsighted eye is to equip it with a diverging lens. Since the light is focused in front of the retina, a diverging lens will serve to diverge light before it reaches the eye. This light will then be converged by the cornea and lens to produce an image on the retina.
When sunlight passes through the window of the classroom, the path of the light is visible. What is this effect called? Give reason.
What is the cause of Presbyopia?
Presbyopia is the gradual loss of eyes' ability to focus on nearby objects. It is caused by an age-related process. Presbyopia generally is believed to grow from a gradual thickening and loss of flexibility of the natural lens inside human eye. These age-related changes occur within the proteins in the lens, making the lens harder and thus it loses its elasticity over time.
The most common correction for Presbyopia is the use of eyeglasses with bifocal lens.
Least distance of distinct vision of a person is observed as 35 cm. What lens is useful for him to see his surroundings clearly ? Why ?
The optical prescription of a pair of spectacle is - Right eye : - 3.5 D, Left eye : - 4.00 D
i) Name the defect of the eye
ii) Are these lenses thinner at the middle or at the edges?
iii) Which lens has a greater focal length?
a. Draw the given diagram and label the following in the diagram.
i) Incident ray
ii) Refracted ray
iii) Emergent ray
iv) Angle of refraction
v) Angle of deviation
vi) Angle of emergence
b. The refractive index of diamond is 2.What is the meaning of this statement in relation to the speed of light?
Given: Refractive index of diamond is $$2.42$$
Solution:
We know , $$\mu = \dfrac{c}{v}$$
Hence,
$$\text{Refractive index of diamond} = \dfrac {\text{Speed of light in air}}{\text{Speed of light in diamond}}\\\implies \text{Speed of light in air}=2.42\times \text{Speed of light in diamond}$$
This means the speed of light in air is $$2.42$$ times the speed of light in diamond.
If the far point of a myopic person is $$75 \ cm$$, what should be the focal length of the lens used to rectify this defect?
Why does the sun appear red during sunset and sunrise?
Light from the Sun near the horizon passes
through thicker layers of air and larger distance
in the earth’s atmosphere before reaching our eyes.
However, light from the Sun overhead would
travel relatively shorter distance.
- At noon, the Sun appears white as only a little of the blue and violet colours are scattered.
- Near the horizon, most of the blue light and shorter wavelengths are scattered away by the particles.
A person cannot clearly see objects farther than $$12 \ m$$ from his eye. Name the defect in vision he is suffering from and the lens that should be used to correct this defect.
Explain with the help of a labelled ray diagram, the defect of hypermetropia and how it is corrected.
Hypermetropia or long sightedness : It is the defect of human eye in which a person can see clearly objects at large distances from it. but cannot see nearby objects clearly.
Correction : It is corrected by using spectacles having convex lens, which converges and shifts the image to retina from beyond.
Why red colour is given to signals to be viewed from a distance?
Light from the Sun travels in straight lines. Yet, the light reaches our rooms and brightens it. Name the phenomenon of light that causes this.
Draw the diagram of dispersion of light through prism and show the position of different colours of light obtained on screen.
Dispersion is the separation of light into its constituent colors.
What do you mean by scattering of light?
Blue colour of sky is due to __________.
Define scattering of light.
(a) A person cannot see nearby objects, then write name of eye defect. Write name of lens used to correct this eye defect.
(b) In brief define 'Tyndall effect'.
A. Hypermetropia is the defect of vision in which a person can see distant objects clearly and distinctively, but is not able to see nearby objects comfortably and clearly. In this defect, the rays coming from object converge at point behind the retina. Since a convex lens has the ability to converge incoming rays, it can be used to correct this defect of vision
B. The Tyndall effect, also known as Willis–Tyndall scattering, is light scattering by particles in a colloid or in a very fine suspension.
What is long-sightedness? The near point of a long-sighted person is at $$100 \ cm$$ from his eyes. What would be the power of the lens to be used by him in order to read a book placed at $$25 \ cm$$ from his eyes?
Long-sightedness
If anyone find it difficult to focus on things close up, or if objects near appear blurry, it could be a sign that the person suffers from long sightedness. Distance vision in long-sighted people generally remains clear.
Long sightedness makes reading difficult, particularly if the type is small.
The diagram shows the picture of long or farsightedness and its remedy using converging lens.
Numerical:-
The image and the actual object are on the same side, so the image is considered negative. The real object is at $$25 \ cm$$ away from the front of the lens $$u=-25 \ cm$$, and we want to produce an image that appears to be $$100 \ cm$$ away from the lens $$v=100 \ cm$$. Plug everything into the lens' formula:
$$Power=\dfrac{1}{f}=\dfrac{1}{v}-\dfrac{1}{u}$$
$$Power=\dfrac{1}{0.25(meter)}-\dfrac{1}{1(meter)}$$
Hence, $$\text{Power=3 diopter or 3D}$$
What is myopia (or short sightedness)? What is the cause of this defect? How can this defect be minimized? Explain drawing a ray diagram.
Myopia or short-sightedness : Myopia is a defect in the vision, in which a person can see nearby objects clearly but cannot see the distant objects distinctly after a certain limit. This defect is common among children.
The causes of Myopia are :
i) The eyeball gets elongated along its axis so that the distance between eye lens and retina becomes larger.
ii) Due to excessive curvature of cornea, The focal length of eye lens becomes small.
This defect is minimized by the concave lens of focal length equal to distance of far point from the eye.
Give scientific reason.
The sun appears reddish early in the morning.
A student is unable to see clearly the words written on the black board placed at a distance of approximately $$3$$ m from him. Name the defect of vision the boy is suffering from. State the possible causes of this defect and explain the method of correcting it.
Myopia is that defect of human eye by virtue of which it can see clearly the objects lying at short distance from it. But the far off objects cannot be seen clearly by the myopic eye.
Causes of Myopia : The two possible causes of this defect are :
Increase in the length of the eyeball, as if distance of retina from the eye lens has increased.
Decrease in focal length of the eye lens when the eye is fully relaxed. This is as if the ciliary muscles holding the eye lens do not relax fully and have some tension.
Correction : The image of a distant object (i.e., at infinity) is formed in front of the retina of eye suffering from myopia as shown in figure. (a)
As the image of the object lying at infinity is not formed on the retina of the eye, so such object can not be seen clearly by the myopic eye. The far point of such an eye is near to the eye as shown in figure (b).
This defect can be corrected by using a concave lens of suitable focal length. So, a man suffering from this defect wears spectacles having concave lens of suitable focal length. The concave lens diverges the rays of light entering the eye from infinity. Hence this lens makes the rays of light appear come from the far point O' of the defective eye as shown in figure (c).
Let x = distance of far point of myopic eye, f = focal length of concave lens to be used. As the object to be seen is at infinity and its image is to be formed at the far point, therefore, u and v = –x. Distance of far point O' from eye lens is taken same as the distance of far point O' from concave lens.
from, $$\dfrac{1}{f} = \dfrac{1}{v} - \dfrac{1}{u}$$
or f = -x
Hence, focal length of concave lens used for correcting the myopic eye is equal to distance of far point of the myopic eye.
Why does the sun appear reddish early in the morning? Will this phenomenon be observed by an astronaut on the Moon? Give reason to justify your answer.
Why do stars twinkle? Explain.
Define scattering.
Answer the question in the limit of maximum $$50$$ words.
Describe the dispersion of light by glass prism with neat figure.
When white light is incident on a glass prism it splits into 7 colors broadly known as VIBGYOR ( Violet, Indigo, Blue, Green, Yellow, Orange, Red). The reason behind the dispersion is the dependence of the refractive index on the wavelength of each component light.
State the defect of vision Kavita is suffering from.
Name the type of lens which is used to correct?
i) Myopia ii) Hypermetropia.
Why sun appears red during sunrise & sunset ?
Why does the sky appear white sometimes?
A person with a myopic eye cannot see objects beyond $$1.2\ m$$ distinctly. What should be the type of the corrective lens used to restore proper vision.
With the help of a diagram explain the formation of rainbow in the sky. List two essential conditions for observing a rainbow
Condition required to observe rainbow:
1> Presence of sunlight.
2> Moist air( presence of water molecule in the air)
A doctor has prescribed a corrective lens of power $$+1.5 \ D$$ to a patient. Find the focal length of the lens in centimeter. Is the prescribed lens diverging or converging?
Given,
Power, $$P=1.5D$$
focal length in metres, $$f=\cfrac { 1 }{ P } $$
$$f=\cfrac { 1 }{ 1.5 } m$$
$$f=\cfrac { 2 }{ 3 } m$$
$$=\cfrac { 2 }{ 3 } \times 100 \ cm$$
$$f=66.7 \ cm$$
Since, the power is positive, lens is convex i.e., converging
What is scattering of light?
A ray of white light falls on a prism. Draw ray diagram to show that the prism disperses the white light.
Explain with reason :-
a) Twinkling of stars.
b) Advanced sunset and delayed sunrise.
c) Blue colour of sky.
d) Reddish colour of sky during sunrise and sunset.
With the help of neat and labeled ray diagram explain angle of prism.
Rainbow is an example of dispersion of sunlight.Expalin its formation.
The diagram illustrates what happens to a single ray of sunlight as it passes through a spherical water drop (actual raindrops are not exactly spherical, but the mechanism is still very similar). Upon entering the drop, the light ray is refracted (or bent) into an angle that depends on the wavelength (or color). Blue light is bent more than red light, with green and yellow in between. The diagram uses a red ray and a blue ray to illustrate how the long-wavelength and short-wavelength light is bent into different angles.
What is a Tyndall effect? Write any two application of tyndal effect.
What is the other name for (a) myopia (b) hypermetropia?
What are the difference between dispersion and scattering
What is the scientific name for:(a) near-sightedness(b) far-sightedness
Name of the defect vision which makes the eye-lens its power of accommodation due to old age.
Name the defects of vision in a person:
$$(a)$$ Whose near point is more than 25 cm away.
$$(b)$$ Whose far point is less than infinity.
What do you mean by scattering of light? Write its use in daily life?
What is old age far-sightedness?
Which defect of vision can be rectified by
a) by using a concave lens
b) by using a convex lens
Watching the sunset on a beach, one can see the sun for several minutes after it has 'actually set'. Explain.
The apparent delay in the setting of sun is due to the phenomenon of atmosphere refraction$$.$$
The atmosphere bends sunlight towards the normal$$.$$ This is the reason for the apparent position of the sun remaining above the horizon for some time even when$$,$$ the sun has actual set$$,$$ $$i.e.,$$ gone below the horizon$$.$$
Hence$$,$$ we experience a delayed sunset$$.$$
A boy uses spectacles of focal length $$-50cm$$ .Name the defect of vision he is suffering from.
Name the triangle piece of glass that splits white light into 7 colours.
Explain why the planets do not twinkle?
The red colour of sun during sunrise and sunset is due to _____________ of light.
A beam consisting of red, blue and yellow colors is incident normally on the face AB of an isosceles right angled prism ABC as shown in Fig. Complete the diagram to show the refracted and emergent rays.
$$\large\textbf{Part -A: For Diagram A}$$
In Fig. (a), the ray AB is incident normally, so it does not disperse in the prism but the emergent beam gets dispersed.
$$Conclusion:$$
$$\bullet$$ The completed diagram is shown in Fig-a. The beam is incident normally (∠i$$=$$0°) on the face AB so it passes without deviation (∠r $$=$$ 0°) for all colours and with no dispersion.
$$\bullet$$ Now the refracted beam inside the prism, strikes the face AC at an angle of incidence equal to 45°. Since the refractive index of the prism for the red colour is minimum, hence it deviated least while Blue deviated most.
$$\large\textbf{Part -B: For Diagram B}$$
In Fig. (b), incident ray is not normal to the prism face and hence it gets dispersed both inside and outside the prism as shown.
$$Conclusion:$$
$$\bullet$$ Light rays get dispersed from both faces of the prism hence, we get a wide spectrum of three colours on screen.
$$\bullet$$ Since the refractive index of the prism for the red colour is minimum, hence it deviated least while Blue deviated most.
If in a room we turn the light off then the room suddenly becomes very dark but after some while, it is not very much dark. What is the logic behind this?
Name the defect shown in the diagram.
Define angle of deviation.
Fill in the blank :
The impression of an image lasts on the retina for ............. of a second
Draw a labelled diagram to show (i) white appearance of the sun at noon when it is overhead.
When the sun is overhead at noon, the light travels through the atmosphere and falls perpendicular on earth. Therefore, at noon there is less scattering of the light and all seven components of white light reach the earth. So, the sun appears to be white.
Give scientific reasons:
Stars twinkle but we do not see the twinkling of planets.
Write the structure of eye lens and state the role of ciliary muscle in the human eye.
Draw a labelled diagram to show (i) reddish appearance of the sun at the sunrise or the sunset
When the sun is at horizon, it appears to be reddish because as the light passes through the atmosphere, it enters slant and due to this large amount of scattering takes place. The blue light gets scattered more as compared to red light and the sun appears red.
Refer above image.
What is a rainbow? Draw a labelled diagram to show the formation of a rainbow.
Rainbow is an arch of colours visible in the sky, caused by the refraction and dispersion of the sun's light by rain or other water droplets in the atmosphere. The colours of the rainbow are generally said to be red, orange, yellow, green, blue, indigo, and violet.
Formation :-
when a beam of light enters the prism, it gets refracted ans splits into its seven constituent colours. This splitting of light ray occurs because of the different angles of bending for each other . Hence, each colour passing through the prism bends at different angles with respect to the incident beam.This gives rise to the formation of the colour spectrum.
What do the ciliary muscles do when you are focusing on a nearby object?
Name two parts of the eye which refract light rays (or bend light rays)
What is the principal function of the eye-lens?
Give the scientific names of the following parts of the eye:
(a) Carries signals from an eye to the brain.
(b) Muscles that change the shape of the eye-lens.
(c) A hole in the middle of the iris.
(d) A clear window at the front of the eye.
(e) Changes shape to focus a picture on the retina.
The scientific names of the following parts of the eye are:
(a) Carries signals from an eye to the brain- Optic nerve
(b) Muscles that change the shape of the eye-lens- Ciliary muscles
(c) A hole in the middle of the iris- Pupil
(d) A clear window at the front of the eye- Cornea
(e) Changes shape to focus a picture on the retina- Eye lens
What changes take place in the shape of eye-lens:
(a) when the eye is focused on a near object?
(b) when the eye is focused on a distant object?
Which part of the eye contains cells which are sensitive to light?
What is the function of the lens in the human eye?
A person wears glasses of power $$-2.5$$D. Is the person farsighted or nearsighted? What is the far point of the person without the glasses?
What change is made in the eye to enable it to focus on objects situated at different distances? Illustrate your answer with the help of diagrams.
To focus on distant objects, the ciliary muscles of the eye get fully relaxed and pull the suspensory ligaments attached to the eye-lens tightly. This, in turn, stretches the eye-lens and the eye-lens becomes thin.
To focus on nearby objects, the ciliary muscles of the eyes contract and make the suspensory ligaments loose. The ligaments then stop pulling the eye-lens. The eye-lens bulges under its own elasticity and becomes thick.
To focus on nearby objects, the ciliary muscles of the eyes contract and make the suspensory ligaments loose. The ligaments then stop pulling the eye-lens. The eye-lens bulges under its own elasticity and becomes thick.
Name the part of our eyes which helps us to focus near and distant objects in quick succession.
What is the far point of a person suffering from myopia (or short-sightedness)?
Describe and explain, how a normal eye can see objects lying at various distances clearly.
Name the defect of vision which can be corrected by a diverging lens. Show clearly by a ray diagram how the lens corrects the defect.
Myopia can be corrected by a diverging lens.
Correction of myopia is given in the above diagram.
Correction of myopia is given in the above diagram.
Name the body part with which the terms myopia and hypermetropia are connected.
Ciliary muscles of human eye can contract or relax. How does it help in the normal functioning of the eye?
What is the other name of old age hypermetropia?
Your friend can read a book perfectly well but cannot read the writing on the blackboard unless she sits on the front row in class.
(a) Is she short-sighted or long-sighted?
(b) What type of lenses-converging or diverging-would an optician prescribe for her?
Where is the near point of a person suffering from hypermetropia (or long-sightedness)?
The near-point of a person suffering from hypermetropia is at 50 cm from his eye. What is the nature and power of the lens needed to correct this defect? (Assume that the near-point of the normal eye is 25 cm).
The person needs a convex lens to rectify this defect.
Calculation of power of the lens:
This hypermetropic eye can see the nearby object kept at $$25$$ cm clearly if the image is formed at its own near point i.e. $$50$$ cm.
Object distance, $$ u = -25 \ cm$$
Image distance, $$v = -50 \ cm$$
$$\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$$
Calculation of power of the lens:
This hypermetropic eye can see the nearby object kept at $$25$$ cm clearly if the image is formed at its own near point i.e. $$50$$ cm.
Object distance, $$ u = -25 \ cm$$
Image distance, $$v = -50 \ cm$$
$$\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$$
$$\dfrac{1}{-50} - \dfrac{1}{-25} = \dfrac{1}{f}$$
$$f = +50 \ cm$$
$$P = \dfrac{100}{f(in \ cm)} = \dfrac{100}{50} = +2D$$
Name the defect of vision in a person:
(a) whose near point is more than 25 cm away.
(b) whose far point is less than infinity.
What is presbyopia? Write two causes of this defect. Name the type of lens which can be used to correct presbyopia.
A person can read a book clearly only if he holds it at an arm's length from him. Name the defect of vision:
(a) if the person is an old man
(b) if the person is a young man
State whether the following statement is true or false:
Rabbit has eyes that look sideways.
What is the order of colours in a rainbow, from the outside to the inside?
A person needs a lens of power, $$- 5.5 \ dioptres$$ for correcting his distant vision. For correcting his near vision, he needs a lens of power, $$+1.5 \ dioptres$$. What is the focal length of the lens required for correcting
1. Distant vision, and
2. Near vision?
In a certain murder investigation, it was important to discover whether the victim was long-sighted or short-sighted. How could a detective decide by examining his spectacles?
If the spectacle lenses are convex, the person was long sighted and if the spectacle lenses are concave, the person was short sighted.
How much is our field of view:
(a) with one eye open?
(b) with both eyes open?
Our field of view with
(a) One eye open is - About $$150^\circ $$(b) Both eyes open is - About $$180^o$$
Out of animals of prey and predators, which have their eyes:
At the front of their head?
On the opposite sides of their head?
1. Predators usually have their eyes at the front of their head. Eyes facing front give predators the ability to
focus on and target their prey.
2. Animals of prey have their eyes on the opposite sides of their head. Eyes on the side of the head give prey a larger field of vision. Prey can see more around them, helping prey to notice predators that may be sneaking up or approaching them.
2. Animals of prey have their eyes on the opposite sides of their head. Eyes on the side of the head give prey a larger field of vision. Prey can see more around them, helping prey to notice predators that may be sneaking up or approaching them.
Fill in the following blanks with suitable words:
A person is short-sighted if his eyeball is too ......... Spectacles with a .......... lens are needed.
A person is long-sighted if his eyeball is too ......... Spectacles with a .......... lens are needed. These focus light rays exactly on to the .......
A short-sighted person has a near point of 15 cm and a far point of 40 cm.
(a) Can he see clearly an object at a distance of:
(i) 5 cm ? (ii) 25 cm ? (iii) 50 cm ?
(b) To see clearly an object at infinity, what kind of spectacle lenses does he need?
Explain why, the sun can be seen about two minutes before actual sunrise. Draw a diagram to illustrate your answer.
State whether the following statement is true or false:
The planets twinkle at night due to atmospheric refraction of light.
Draw a diagram to show how white light can be dispersed into a spectrum by using a glass prism. Mark the various colours of the spectrum.
When white light is passed through a glass prism it splits into its spectrum of colours (in order violet, indigo, blue, green, yellow, orange and red) and this process of white light splitting into its constituent colours is termed as dispersion.
Because different color travel with different speed in prism they split while moving in prism.
Why do you not see a spectrum of colours when light passes through a flat pane of glass?
Name the phenomenon which causes the twinkling of stars.
Why do stars seem higher than they actually are? Illustrate your answer with the help of a diagram.
What is tyndall effect? Explain with an example
The scattering of light by particles in its path is called Tyndall effect.
Ex. When a beam of sunlight enters a dusty room through a window, then its path becomes visible to us due to the scattering of the light by the dust particles present in the air.
Ex. When a beam of sunlight enters a dusty room through a window, then its path becomes visible to us due to the scattering of the light by the dust particles present in the air.
State two effects produced by the scattering of light by the atmosphere.
Fill in the following blanks with suitable words:
We can see the sun about ........ minutes before the actual sunrise and about ........ minutes after the actual sunset because of atmospheric .......
By how much time the day would have been shorter if the earth had no atmosphere?
The atmospheric refraction causes an early sunrise and delayed sunset. So we are able to see the sun $$2\ minutes$$ before the sun actually rise and $$2\ minutes$$ after the sun is set. Therefore, the presence of the atmosphere allows us to see the sun for $$4\ minutes$$ extra.
If there is no atmosphere then we will not be able to see the sun for extra $$4\ minutes$$. Therefore, the day would be shorter by $$4\ minutes$$.
In an experiment to study the scattering of light by passing a beam of white light through a colloidal solution of sulphur in a transparent glass tank:
(a) Which colour is observed from the front of the glass tank? Does this colour correspond to the colour of sky on a clear day or the colour of sky around the sun at sunset?
(b) Which colour is observed from the sides of the glass tank? Does this colour correspond to the colour of sky on a clear day or the colour of sky around the sun at sunset?
Why does the sky appear blue on a clear day?
Why does the sun appear red at sunrise?
During sunrise, the sun is near the horizon. At this time the sunlight has to travel a longer distance through the atmosphere to reach our eyes. During this advancement, the sunlight gets scattered by the minute particles in the atmosphere. As the wavelength of the blue light is shorter, it scatters the most and can't reach us. The wavelength of red light is comparatively longer and hence it suffers less scattering. Therefore, the light that is reaching us contains mostly the red colour. That is why the sun appears red during sunrise.
The ciliary muscles of a normal eye are in their (i) most relaxed (ii) most contracted state. In which of the two cases is the focal length of the eye-lens more?
What is meant by 'persistence of vision'? We are able to see movie picture in a cinema hall. How does this happen?
Draw a ray diagram showing the dispersion through a prism when
a narrow beamof white light is incident on one of its refracting surfaces. Also,
indicate the order of thecolours of the spectrum obtained.
Order of colours from bottom to top: Violet, Indigo, Blue, Green, Yellow, Orange and Red. One can memorise these colours by remembering keyword $$\text{VIBGYOR}$$
Can an observer see a rainbow on the moon?
News spread in the school that a student of Class IX-A has expired due to heart attack, but he has donated his beautiful eyes to one of his friends. All the members of school felt very sad for his untimely death, but on the other hand they were overwhelmed on hearing the donation of his eyes to his friend, who would now be able to see this beautiful nature.
Answer the following questions based on the situation given above.
(i) Do you think that the student who expired had done good job? Is it worth to donate vital organs?
(ii) What values are promoted here?
(iii) What other organs can be donated after death?
Is the position of a star as seen by us its true position? Justify
your answer.
No, position of start as seen by us is not true position of star.
This is due to fact that The density of atmospheric layers increases as we move from top to bottom. Due to this,starlight bends towards the normal as it passes through different layers of atmosphere. Due to this, the apparent position of star is a little above its actual position in sky.
Why do we see a rainbow in the sky only after rainfall?
Formation of rainbow is only possible when dispersion of light takes place through a suitable surface. After rainfall, some raindrops remain in the clouds. Moreover, the opposite side of sky works like a screen on which rainbow is formed. Hence, rainbow is seen in the sky only after rainfall.
How are we able to see nearby and also the distant objects clearly?
A person needs a lens of power -4.5 D for correction of her vision.
(a) What kind of defect in vision is she suffering from?
(b) What is the focal length of the corrective lens?
(c) What is the nature of the corrective lens?
Explain the structure and functioning of the human eye. How are we able to see nearby as well as distant objects?
The human eye has the following main parts:
Cornea: Human eye is spherical in shape. It has a tough white coat that protects the interior of the eye. The front portion of this coat is transparent and is called the cornea.
Iris: This is a dark muscular structure behind the cornea. The unique colour of a person's eye is because of the colour of the iris.
Pupil: The small opening in the iris is called the pupil. Iris controls the size of the pupil and thus controls the amount of light entering the eye. Light enters the eye through the pupil.
Lens: Lens is thicker in the middle and is made of transparent material. The lens focuses the light on the back of the eye; called the retina.
Retina: The back of the eye is called the retina. It works like a screen; on which an image is formed.
Rods and cones function as photoreceptors in the retina of the eye, turning visible light into neuronal signals, which are sent to the brain. This process is called transduction. These nerve cells are connected to the optic nerve.
Formation of Image in Eye: Light rays enter the eye through the pupil and pass through the lens. The lens focuses light rays on the retina. The real, inverted and smaller image is formed on the retina. The optic nerve carries the message to the brain. The brain interprets the message and we get the sense of vision.
A beam of white light falling on a glass prism gets split up into seven colours marked $$1$$ to $$7$$ as shown in the diagram.
A student makes the following statements about the spectrum observed on the screen.
(a) The colours at positions marked $$3$$ and $$5$$ are similar to the colour of the sky and the colour of gold metal respectively.
Is the above statement made by the student correct or incorrect? Justify.
(b) Which two positions correspond closely to the colour of (i) brinjal, (i) 'danger' or stop signal lights?
Mention any four phenomena that can be observed due to atmospheric refraction of light on the earth.
How can we explain the reddish appearance of sun at sunrise or sunset? Why does it not appear red at noon?
Colours near the red end of the spectrum scatter the least. This
happens because of short wavelength of reddish colours. During sunset and sunrise,
sunlight needs to travel more distance to reach us, Red colour is able to reach
us because it is scattered the least.Hence, sky appears reddish during sunrise/sunset.
The sky appears reddish during sunrise/sunset but it appears white
at noon. During noon, the sunlight has to travel less distance to reach us. Most
of the colours reaching us get scattered. Due to this, sky appears white at noon.
A myopic adult has a far point at $$0.1m$$. His power of accommodation is $$4$$ diopters
(i) What power lenses are required to see distant objects
(ii) What is his near point without glasses?
(iii) What is his near point with glasses? (Take the image distance from the lens of the eye to the retina to be $$2cm$$)
Explain the phenomenon of
dispersion of white light through a glass prism, using suitable ray diagram.
When ray of light enters a prism, it bends because of refraction
of light. When the ray of light finally emerges out of the prism, it deviates drastically from its original path. This happens because of unique shape of prism.
Different colours in the visible spectrum have different speeds.
Due to this, different colours bend at different angles of deviation. As a result,
the emergent light appears as a band of seven colours; the colours which are the components of white light. These colours are Violet, Indigo, Blue, Green, Yellow, Orange and Red.
Segregation of white light into its different components is called
dispersion of light.
The color of sky in direction other than of the sun is blue. Explain:
As the light travels through the atmosphere, it gets scattered in different directions by the air molecules present in its path. The blue light due to its short wavelength is scattered more as compared to the red light of long wavelength. Thus the light reaching our eye directly from the sun is rich in red color, while the light reaching our eye from all other directions is the scattered blue light. Therefore, the sky in a direction other than in the direction of sun is seen blue.
When do we consider a person to be myopic or hypermetropic? Explain
using diagrams how the defects associated with myopic and hypermetropic eye can
be corrected?
When a person is unable to clearly see distant objects, he is
considered a myopic person. Such a person is suffering from myopia. This happens when image is formed in front of the retina.
When a person is unable to clearly see a nearby object, he is
considered a hypermetropic person. Such a person is suffering from hypermetropia.
This happen when image is formed behind the retina.
Correction of Myopia: A person suffering from myopia needs to
use a concave lens of suitable focal length. The concave lens diverges the rays coming from infinity. After refraction from the concave lens, the rays appear to be coming from the far point of this person's eye. Due to this, a clear image of distant object is made on the retina of that person.
That is how a myopic person is able to clearly see distant objects;
with the help of suitable concave lens.
Correction of Hypermetropia: A hypermetropic person needs to use
a convex lens of suitable focal length. The convex lens converges the light rays coming from a nearby object.
As a result, these light rays appear to be coming from the near
point of this person's eyes.
Due to this, a clear image of nearby object is made on the retina
of that person. That is how a hypermetropic person is able to clearly see nearby objects; with the help of suitable convex lens.
How does the deviation produced by a triangular prism depend on the colours (or wavelength) of light incident on it ?
The clouds are seen white. Explain.
Will the focal length of a lens for red light be more, same or less than that for blue light?
The diagrams (a) and (b) in figure above show the refractions of a monochromatic ray of light through a parallel sided glass block and a prism, respectively. In each diagram, label the incident, refracted, emergent rays and the angle of deviation.
A convex lens of suitable focal length can rectify which defect of vision?
What holds the crystalline lens in the human eye?
Name the light-sensitive part of the eye where image of an object is formed.
Retina is the light sensitive part of the eye where image of an object is formed.
Match the columns in the following table and explain them.
| Column $$1$$ | Column $$2$$ | Column $$3$$ |
| Farsightedness | Nearby object can be seen clearly | Bifocal lens |
| Presbyopia | Far away object can be seen clearly | Concave lens |
| Nearsightedness | Problem of old age | Concave lens |
Distinguish between presbyopia and hypermetropia.
What is the direction of rainbow formation? What is the position of red colour in the rainbow?
In which type of eye defect , the near point of the eye becomes more than $$ 25 cm? $$
Give reason for the early sunrise and the delayed sunset
Give the difference between myopia and hypermetropia
Difference between Myopia and Hypermetropia is as given in above table.
The far point of a myopic person is 83 cm in front of the eye. What is the nature and power of the lens required to correct the problem?
What is the scattering of light ? On what factors does it depend ?
What causes presbyopia? Name the lens to correct this defect.
What is the reason for Advance sunrise and delayed sunset?
Which colour deviates the least?
The color red deviates the least and color violet deviates the most.
In the experiment using hypo solution to demonstrate scattering of light, when hydrochloric acid was added to the solution, which colour did spread first in the solution?
We find fine microscopic sulphur particles precipitating in about $$2$$ to $$3$$ minutes when hydrochloric acid was added to the solution. As the sulphur particles begin to form, we can observe
the blue light from the three sides
of the glass tank. This is due to
scattering of short wavelengths by
minute colloidal sulphur
particles.
Write down what else can be done to minimize the light pollution.
Make Newtons disc by painting the constituent colours of white col-our of white light in the same order and proportion.
The disc appeared white due to the persistence of vision. Find out more examples of the persistence of vision and write them down.
Where will be the sun when the rainbow is seen in the west?
In the east because rainbows are formed when sunlight is scattered from raindrops into the eyes of an observer. Hence, the sun has to be present to the back side of the observer.
Most raindrops are spherical rather than the often depicted 'teardrop' shape and it is this spherical shape that provides the conditions for a rainbow to be seen.
Which component colour in white light does undergo maximum scattering in the atmosphere?
The western horizon remains reddish for some more time even after sunset. Why?
During sunset, the sunlight travels a maximum distance through the atmosphere to reach our eyes. Because of the long-distance, rays having bigger wavelength scatter away, whereas red colour rays have smaller wavelength so they travelled longer distance without scattering. So the maximum amount of red colour rays observe to our eyes So the horizon appears red. (see fig-I)
Also, sun appears approx 2 minutes after the sunset because when the light rays come from the sun to earth, optical density of medium goes increase and rays bend to the normal (According to Snell's law). Due to reflection from atmospheric medium, a virtual image forms above the horizon up to 2 mint of sunset. (see fig-II)
Fill in the blanks :
After the rain when the sun is low in the sky the ................ appears in the Sky.
How many colours are there in Rainbow?
The image formation of a defected eye is given below
a. In which position images formed on a normal eye?
b. What is this eye defect?
c. How to solve this defect? Draw the diagram.
a. On the retina.
b. Near-sightedness(Myopia).
c. Suitable power of concave lenses is used to solve this problem.
Why does the path of light seen when dust particles are spread in cinema theatre and smart room.
Find the appropriate terms from the box for the statements given below.
Red, Scattering, Refraction, Dispersion Phenomenon responsible for the Tyndal effect.
Under what conditions is a mirage formed? While driving on a hot day, sometimes you see what appears to be water on the road far ahead. When you arrive at the location of the water, however, the road is perfectly dry. Explain this phenomenon.
Analyse the following statements and find out the reason behind them.
Raindrops falling down during rain appear like a glass rod.
Analyse the following statement and find out the reason behind it.
A rainbow is seen in the shape of a full circle when viewed from a height.
A person sees clearly wearing eyeglasses that have a power of -4.00 diopters when the lenses are 2.00 cm in front of the eyes. (a) What is the focal length of the lens? (b) Is the person nearsighted or farsighted?(c) If the person wants to switch to contact lenses placed directly on the eyes, what lens power should be prescribed?
What is a mirage? How is it formed?
Mirage is an optical phenomenon that creates an illusion of the presence of water and is a result of the refraction of light from a non-uniform medium. Mirage is observed mainly during sunny days when driving on a roadway.
$$\underline{\text{Formation of mirage:}}$$
When the light rays from the sun travel straight through the atmosphere, these light rays travel at different speeds through the hot and cold air. For mirage to occur the ground should be very hot and the surrounding air cold. The layer which is above the ground gets warmed. The light ray gets refracted when the light moves through the cold air and into the hot air layer. This is how mirage occurs.
A child sitting at the backbench of a classroom is unable to see the letters on the board clearly. What is the defect of the eye of the child? How can it be remedied? Draw its ray diagram.
The child is suffered from Myopia or Nearsightedness. This can be rectified by using a concave lens of suitable power.
Find the appropriate terms from the box for the statements given below.
Red, Scattering, Refraction, Dispersion
Phenomenon responsible for Rainbow.
A person is advised to wear spectacles with convex lenses. What type of defect of vision is he suffering from ?
The far point of a myopic person is $$80$$ cm in front of the eye. What is the nature and power of the lens required to correct the problem?
A certain child’s near point is $$10.0 \ cm$$; her far point(with eyes relaxed) is $$125 \ cm$$. Each eye lens is $$2.00 \ cm$$ from the retina. (a) Between what limits, measured in diopters, does the power of this lens–cornea combination vary? (b) Calculate the power of the eyeglass lens the child should use for relaxed distance vision. Is the lens converging or diverging?
(a) when child clearly sees objects at her far point the lens cornea combination has assumed a focal length suitable for forming the image on the retina. Using the lens equation ,
$$\dfrac{1}{f} = \dfrac {1}{v} - \dfrac {1}{u} $$, Also power $$P=\dfrac{1}{f}$$
$$\dfrac{1}{f} = \dfrac {1}{v} - \dfrac {1}{u} $$, Also power $$P=\dfrac{1}{f}$$
Here, $$u=-125 \ cm = -1.25 \ m$$ and $$v=2 \ cm = 0.02 \ m$$
Similarly, when eye is focused on object at near point, the optical power of lens cornea combination is
$$P_{near} = \dfrac{1}{f} = \dfrac {1}{0.1} + \dfrac {1}{0.020} = +60 \ Diopters$$
Hence, the range of the power of lens cornea combination is $$+ 50.8 \ Diopters \leq P \leq 60.0 \ Diopters$$
(b) if child sees very distant object clearly, her eyeglass lens must form an erect , virtual image for the object at infinity at the far point of her eye, optical power of required lens is,
$$P = \dfrac{1}{u}+\dfrac{1}{v} = 0 + \dfrac {1}{-0.125} = -0.8000 \ Diopters$$
$$P_{far} = \dfrac {1}{f} = \dfrac {1}{1.25} + \dfrac {1}{0.02} = +50.8 \ Diopters$$
Similarly, when eye is focused on object at near point, the optical power of lens cornea combination is
$$P_{near} = \dfrac{1}{f} = \dfrac {1}{0.1} + \dfrac {1}{0.020} = +60 \ Diopters$$
Hence, the range of the power of lens cornea combination is $$+ 50.8 \ Diopters \leq P \leq 60.0 \ Diopters$$
(b) if child sees very distant object clearly, her eyeglass lens must form an erect , virtual image for the object at infinity at the far point of her eye, optical power of required lens is,
$$P = \dfrac{1}{u}+\dfrac{1}{v} = 0 + \dfrac {1}{-0.125} = -0.8000 \ Diopters$$
Since the power and hence the focal length of this lens is -ve, it is diverging.
(i) Show the formation of the rainbow with the help of a ray diagram.
(ii) What are the conditions to observe the rainbow?
(i) Refer to Figure.
(ii) The sun comes out after a rainfall and tiny droplets present in the atmosphere act as naturally formed prisms.
The observer of the rainbow must be present in between the rainbow and the sun (source of light) such that the sun is behind the observer.
Given an example of optical phenomenon where Tyndall effect can be observed.
A person is advised to wear spectacles with concave lenses. What type of defect of vision is he suffering from ?
The person is suffering from myopia. It is type of vision defect, in which a person is able to see nearby objects clearly but distant objects appear blurry. In this defect, the rays coming from distant objects focus in front of the retina and a concave lens is used to correct this.
Name the atmospheric phenomenon due to which the sun can be seen above the horizon about two minutes before actual sunrise.
The accommodation limits for a near-sighted person’s eyes are $$18.0 \ cm$$ and $$80.0 \ cm$$. When he wears his glasses, he can see faraway objects clearly. At what minimum distance is he able to see objects clearly?
A convex lens made of glass forms a sharp image on the screen for a particular position of an object with respect to the lens. A human eye lens is also a convex lens but it can form sharp images on the retina of eye for different positions of the objects. Explain, why?
A $$52$$-year old near-sighted person wears eye-glass with a power of $$5.5$$ dioptres for distance viewing. His doctor prescribes a correction of $$+1.5$$ dioptres in the near-vision section of his bi-focal. This is measured relative to the main part of the lens (i) What is the focal length of his distance-viewing part of the lens? (ii) What is the focal length of the near-vision section of the lens?
What is the defect of vision in the human eye? Vishal unable to see the far object clearly. So which type of defect of vision does it posses, explain it.
A $$14$$-year old student is not able to see clearly the questions written on the blackboard placed at a distance of $$5\ m$$ from him.
(a) Name the defect of vision he is suffering from.
(b) With the help of labelled ray diagrams show how this defect can be corrected.
(c) Name the type of lens used to correct this defect.
$$(a)$$ The defect of vision he is suffering from is called short -sightedness (Myopia).
$$(b)$$ Refer Image. This defect can be corrected using a diverging lens.
$$(c)$$ The type of lens used to correct this defect is concave Lens.
A star sometimes appears brighter and some other times fainter. What is this effect called? State the reason for this effect.
(a) What is meant by the power of accommodation of an eye?
(b) A person with a myopic eye cannot see objects beyond $$1.2\ m$$ directly. What should be the type of the corrective lens used? What would be its power?
A person with a myopic eye cannot see objects beyond $$1.2\ m$$ directly. What should be the type of corrective lens used? What would be its power?
(a) A student cannot see clearly a chart hanging on a wall placed at a distance $$3\ m$$ from his eye. Name the defect of vision he is suffering from. Draw a ray diagram to illustrate this defect. List its two possible causes.
(b) Draw a ray diagram to show how this defect may be corrected using a lens of appropriate focal length.
(a) Myopia or Short-sightedness is the inability of an eye in viewing long distance objects. The image in this case falls before the retina. For every myopic eye, there exists a far point beyond which clear image cannot be seen. Short-sightedness is caused due to
(i) excessive curvature in cornea.
(ii) elongation of eyeball.
(b) The short-sightedness is corrected by using a concave lens which diverges and shifts the image to the retina.
What is hypermetropia (far-sighted-ness)? Draw a ray diagram to show how this defect can be corrected using a lens.
Hypermetropia is the inability of an eye in viewing the nearby objects. The image in this case is formed beyond the retina.This is due to the reason that the converging power of eye lens decreases and it is not able to converge the diverging rays coming from nearby objects to retina.
So It is corrected by using a convex lens, due to this net converging power is increased ,which converges and shifts the image to the retina.
Explain why do the planets not twinkle but the stars twinkle.
Planets, being of larger size can be taken as a collection of large number of point-sized objects/sources of light, which nullify the twinkling effect of each other. Due to varying conditions of the atmosphere, starlight undergoes multiple refraction and its path varies slightly while passing through the atmosphere. Therefore, the apparent position of the star fluctuates and the amount of light entering the eye changes continuously. The star sometimes appears brighter and some other time, it appears fainter. This causes twinkling of stars.
In the figure given below, a narrow beam of white light is shown to pass through a triangular glass prism. After passing through the prism, it produces a spectrum $$XY$$ on the screen.
(i) Name the phenomenon.
(ii) State the colours seen at $$X$$ and $$Y$$.
(iii) Why do different colours of white light bend at different angles through a prism?
Hint - Using dispersion of light through the Prism
Mirage is an example of _____
Solve the following crossword with the help of the given clues:
Across
(1) A microscope consisting two convex lenses
(3) The jelly-like substances that fills the area of the eyeball between the retina and the lens.
(5) A lens mounted in a frame used to make an object appear layer than it really is
(7) The organ of the body with which humans are able to see
(9) The clear, watery fluid that fills the chamber of the eye between the cornea and the lens.
(11) An optical instrument used to view distant objects. It makes far away objects appear larger when viewed through it.
(12) Defect of human eye, by virtue of which a person can see clearly the distant objects but we the nearby objects.
Down
(1) The front transparent part of the eye that is bulged outwards. Light from objects enter the eye through it.
(2) An optical device consisting of a conve lens of small focal length that is used to make things.
(4) An apparatus for taking photographs.
(6) The flat-coloured membrane in the lens that contacts the size of the pupil
(8) The opening in the centre of the iris through wheels light enters the eye.
(10) The defect of human eye due to which a person is able to near objects at close range better than distant objects.
Across
(1) Compound microscope.
(3) Vitreous humour.
(5) Magnifying glass.
(7) Human eye.
(9) Aqueous humour.
(11) Telescope.
(12) Hypermetropia.
Down
(1) Cornea.
(2) Simple microscope.
(4) Camera.
(6) IRIS.
(8) PUPIL.
(10) MYOPIA.
(1) Compound microscope.
(3) Vitreous humour.
(5) Magnifying glass.
(7) Human eye.
(9) Aqueous humour.
(11) Telescope.
(12) Hypermetropia.
Down
(1) Cornea.
(2) Simple microscope.
(4) Camera.
(6) IRIS.
(8) PUPIL.
(10) MYOPIA.
In a Std. X class, out of $$40$$ students, $$10$$ students use spectacles, $$2$$ students have positive power and $$8$$ students have negative power of lenses in their spectacles.
Answer the following questions :
(i) What does the negative power indicate?
(ii) What does the positive power indicate?
(iii) Generally which type of spectacles do most of the students use?
(iv) What defect of eyesight do most of the students suffer from?
(v) Give two possible reasons for the above defect.
Mirage formation is due to _____
A student finds the writing on the blackboard as blurred and unclear when sitting on the last desk in the classroom. He however, sees it clearly when sitting on the front dest at an approximate distance of 2 m from the blackboard. Draw ray diagrams to illustrate the formation of image of the blackboard writting by his eye - lens when he is seated at the (i) last desk (ii) front desk
Name the kind of lens that would help him to see clearly even when he is seated at the last desk. Draw a ray diagram to illustrate how this lens helps him to see clearly.
Student is suffering from myopia. Student needs to use concave lens here. It will form a virtual object at 2m (Far point) away from eye.
(a) A student suffering from myopia is not able to see distinctly the objects placed beyond $$5 m$$. List two possible reasons due to which this defect of vision may have arisen. With the help of ray diagrams, explain
(i) why the student is unable to see distinctly the objects placed beyond $$5m$$ from his eyes.
(ii) the type of the corrective lens used to restore proper vision and how this defect is corrected by the use of this lens.
(b) If, in this case, the numerical value of the focal length of the corrective lens is $$5m$$, find the power of the lens as per the new Cartesian sign convention.
(a) Myopia is also known as near-sightedness. A person with myopia can see nearby objects clearly but cannot see distant objects distinctly. A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres. In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself. This defect may arise due to: ($$1$$) excessive curvature of the eye lens, or ($$2$$) elongation of the eyeball.
(i) The student is able to see the objects clearly upto $$5m$$, i.e., his far point is $$5m$$. For a myopic eye with the near point as $$5m$$, if any object is kept beyond this distance, the image is not formed on the retina. Instead, the image is formed in front of the retina as shown in the figure. Hence, the student is unable to see distinctly the object places beyond $$5m$$ from his eyes.
(ii) This defect can be corrected by using a concave lens of suitable power. A concave lens of suitable power will bring the image back on to the retina and thus the defect is corrected.
(b) Given, focal length $$(f) = -5m$$
$$Power (P) = \cfrac { 1 }{ focal \ length (f) } = \cfrac { 1 }{ -5m } = -0.2 D$$
Explain with the help of a labelled ray diagram the defect of myopia and how it is corrected.
Myopia is a defect of vision in which a person clearly sees all the nearby objects, but is unable to see the distinct object comfortably and his eye is known as myopia eye. A myopic eye has its far pint near that infinity. It forms the image of an distinct object in front of the retina as shown in the figure A.
Myopia can be caused by
1. increase in curvature if the lens.
2. increase in the length if the eye ball.
Since concave lens has an ability to diverge incoming rays, it us used to correct this defect of vision. The image is allowed to form at thee retina by using a concave lens of suitable power as shown in the figure B
State the defects of vision in human eyes and its remedies. Explain the defect of myopia and remedies to control it.
The inability of the ciliary muscles to change the thickness of eye-lens as per requirement and to form the image of an object on the retina leads to the defect of vision.
The three main common defects of vision are as follows:
- Myopia or near-sightedness (Corrected by concave lens)
- Hypermetropia or far-sightedness (Corrected by convex lens)
- Presbyopia (Corrected by bifocal lens)
Myopia or near-sightedness: It is the inability of the eyes to see distant objects clearly. It arises when the eye-lens does not become thin as per requirement and as such the image of the object is formed at a distance shorter than the retina. Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.
a) Complete Fig. A and Fig. B indicating the changes the incident rays undergo as they pass through and come out of the prisms.
b) On completion if there is any difference between A and B. Mention the reason for it.
a) The explanation is given in the diagram.
b) Since a laser ray is monochromatic there is no dispersion. Sunlight is composite light consisting of different colours. Hence, it undergoes dispersion. Violet colour with low wavelength is most deviated than the longer wavelength red colour.The reason for dispersion is different velocity of different colors while moving in the prism.
You know that myopia is a common refractive defect of vision. Person with this defect can see only nearby objects. Using concave lens of suitable power this defect is corrected.
(a) Mention other two types of defects like this.
(b) Explain how we can correct it.
The given diagram is showing a defect of the human eye. Study it and answer the following question: Name the defect shown in the figure.
What is myopia? How does it occur? How can it be corrected? Explain with figure.
Myopia is the term used to define short-sightedness. Light from a distant object forms an image before it reaches the retina. This defect may arise due to (i) excessive curvature of the eye lens, or (ii) elongation of the eyeball.
A myopic person has a clear vision when looking at objects close to them, but distant objects will appear blurred.
Myopia is easily corrected at your local optician using prescription glasses or contact lenses specifically designed to counteract the effect. A concave lens (minus powered) is placed in front of a myopic eye, moving the image back to the retina and clarifying the image.
What is hypermetropia? What are the possible reasons of hypermetropia? How is hypermetropia corrected? Explain with figures.
(a) Hypermetropia or Long Sightedness is an eye defect. A person suffering from it can see distant objects clearly but is not able to see nearby objects clearly.
(b) Hypermetropia is caused due to slight flattening of the eye ball and the image is formed behind the retina.
(c) The weak action of the ciliary muscles causes low converging power of eye lens.
(d) The distance between eye lens and the retina decreases due to the flattening or shortening of the lens. This increases the focal length of the eye lens.
(e) This defect can be corrected by wearing spectacles having a convex lens which are of suitable focal length. The power of convex lens depends upon the degree of defect in the eye, it varies from person to person. The power of spectacles for a hypermetropic eye is positive because the focal length of the convex lens is positive.
(b) Hypermetropia is caused due to slight flattening of the eye ball and the image is formed behind the retina.
(c) The weak action of the ciliary muscles causes low converging power of eye lens.
(d) The distance between eye lens and the retina decreases due to the flattening or shortening of the lens. This increases the focal length of the eye lens.
(e) This defect can be corrected by wearing spectacles having a convex lens which are of suitable focal length. The power of convex lens depends upon the degree of defect in the eye, it varies from person to person. The power of spectacles for a hypermetropic eye is positive because the focal length of the convex lens is positive.
What are myopia and hypometropia? How are they corrected, explain using a diagram.
A person with a hypermetropia eye cannot see objects beyond $$1$$ metre directly. What should be the type of the corrective lens used? What would be its power?
Complete the following sentence :(I). A short-sighted person cannot see ............... object clearly. Short-sightedness can be correct by using ........... lenses.(II). A long-sighted person cannot see .............. object clearly. Long-sightedness can be corrected by using ............ lenses.
The range of vision of a person is $$50\ cm - 120\ cm$$ which defect of vision is he suffering from?
A student sitting in the last row of the class-room is not able to read clearly the writing on the blackboard. (I) Name the type of defect he suffering from.(II) How can these defect by corrected?
A man is suffering from myopia, cannot see objects beyond $$10.0\ m$$ distinctly. What should be the type of corrective lens used to restore proper vision? Find the focal length and power of the lens?
Your friend can read a book perfectly well but cannot read the writing on the blackboard unless she sits on the front row in class. Is she short-sighted or long-sighted?
The near point of a hyper metropic person is $$50$$cm from the eye. What is the power of the lens required to enable the person to read clearly a book held at $$25$$cm from the eye?
Define Tyndall effect?
The earth’s atmosphere is a mixture of smoke particles, tiny water droplets and air particles. When light falls on such colloidal particles, it is scattered (or deflected) thereby making the path of light beam visible. This phenomenon is called Tyndall effect. The light rays reach us after the deflection (scattering) from all these particles. Tyndall effect is also seen due to scattering of light through tiny droplets in mist or when sunlight enters a canopy of dark forest(GIVEN IN PICTURE)
You are given that the diameter of the eyeball is about $$2.3$$ cm and a normal eye can adjust the focal length of its eye lens to see objects situated anywhere from $$25$$ cm to an infinite distance away from it.What is the power of the (normal) eye lens, when ciliary muscles are fully relaxed?
What is the power of the (normal)eye lens, when ciliary muscles are in their maximum contract position?
The maximum variation in the power of the eye lens, when it adjusts itself, from the normal relaxed position to the position where the eye can see the nearby object clearly?
Name two phenomenon responsible for rainbow formation
(i) What is myopia? state the two cause of myopia. With the help of labelled ray diagrams show
(a) the eye defect myopia
(b) correction of myopia using a lens.
(ii) Why is the normal eye unable to focus on an object placed within $$10\,cm$$ from the eye?
This is the defect of eyes in which the person has difficulty in seeing distant objects but can easily see nearby objects. This eye defect is also called as nearsightedness.
For the persons with Myopia, the far point is nearer than infinity. The image of the object placed at a distance is formed in front of the retina and not on the retina.
It is caused by:
1) Excessive curvature of the eye lens
2) Elongation of the eyeball
A myopic eye can be cured by using a concave lens of suitable power.
The ciliary muscles of our eyes are unable to contract after a certain point.
If the object is placed at a distance less than 10 cms from our eyes, then it will be reflected beyond the retina making it blurry for the viewer.
This happens because the light rays emitted are emitted to a point other than the focal point.
Explain the blue appearance of sky ?
How the eye of a person suffering from myopia can be corrected? Explain with the help of a ray diagram.
The ray diagram below shows the myopic defect and its correction method. A concave lens is used for the correction of myopia.
(a) What is short-sightedness? State the two causes of short-sightedness (or myopia). With the help of ray diagrams, show:
the eye-defect short-sightedness.
correction of short-sightedness by using a lens.
(b) A person having short-sightedness cannot see objects clearly beyond a distance of 1.5 m. What would be the nature and power of the corrective lens used to restore proper vision?
(a) Short Sightedness is that defect of vision due to which a person can see nearby objects clearly but cannot see distant objects clearly.
Causes:
1. Excessive curvature of the eye lens
2. Elongation of the eyeball
Ray diagram for the below two are given above.
(i) eye-defect short sightedness
Causes:
1. Excessive curvature of the eye lens
2. Elongation of the eyeball
Ray diagram for the below two are given above.
(i) eye-defect short sightedness
(ii) correction of short-sightedness by using a lens.
(b) Concave lens should be used to restore proper vision.
Calculation of power:
Object distance, $$ u = -\infty$$
Image distance, $$v = -1.5 \ m$$
$$\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$$
$$\dfrac{1}{-1.5} - \dfrac{1}{-\infty} = \dfrac{1}{f}$$
$$f = -1.5 \ m$$
$$P = \dfrac{1}{f} = \dfrac{1}{-1.5} = -0.67D$$
(b) Concave lens should be used to restore proper vision.
Calculation of power:
Object distance, $$ u = -\infty$$
Image distance, $$v = -1.5 \ m$$
$$\dfrac{1}{v} - \dfrac{1}{u} = \dfrac{1}{f}$$
$$\dfrac{1}{-1.5} - \dfrac{1}{-\infty} = \dfrac{1}{f}$$
$$f = -1.5 \ m$$
$$P = \dfrac{1}{f} = \dfrac{1}{-1.5} = -0.67D$$
(a) What is atmospheric refraction? What causes atmospheric refraction?
(b) Why do stars twinkle on a clear night?
(c) Explain why, the planets do not twinkle at night.
Name the defect of vision which can be corrected by a converging lens. Show dearly by a ray diagram how the lens corrects the defect.
Hypermetropia can be corrected by a converging lens.
Correction of hypermetropia is given in the above diagram:
Correction of hypermetropia is given in the above diagram:
A person suffering from the eye-defect myopia (short-sightedness) can see clearly only up to a distance of 2 metres. What is the nature and power of lens required to rectify this defect?
(a) Draw a diagram to show the refraction of light through a glass prism. On this diagram, mark
incident ray
emergent ray, and
angle of deviation.
(b) What is a rainbow? What are the two conditions necessary for the formation of a rainbow in the sky?
(c) What acts as tiny prisms in the formation of a rainbow?
(d) Name the process which is involved in the formation of a rainbow.
(e) What are the seven colours seen in a rainbow?
(c) Raindrops
(d) Dispersion of light
(e) Red, Orange, Yellow, Green, Blue, Indigo and Violet.
Explain with the help of labelled ray-diagram, the defect of vision called hypermetropia, and how it is corrected by a lens.
Hypermetropia (or long-sightedness) is that defect of vision due to which a person cannot see the nearby objects clearly (though he can see the distant object clearly). This eye defect can be corrected by using a convex lens.
Hypermetropic eye and its correction is given in the following diagrams:
Hypermetropic eye and its correction is given in the following diagrams:
We know that light refracts (or bends) when it goes from one medium to another. Now, the atmosphere contains only air. Then how does light get refracted on passing through only air in the atmosphere?
(a) Draw a neat and labelled diagram of the experimental set up for observing the scattering of light in a colloidal solution of sulpher to show how the sky appears blue, and the sun appears red at sunrise and sunset.
(b) Out of blue light and red light, which one is scattered more easily?
(c) Which component of sunlight is scattered away when the sun appears red at sunrise or sunset?
(d) What causes the scattering of blue component of sunlight in the atmosphere?
(b) Blue light scatters more easily due to its smaller wavelength.
(c) Shortest wavelength component i.e. blue light scatters away when the sun appears red at sunrise or sunset.
(d) Gas molecules present in the air.
(c) Shortest wavelength component i.e. blue light scatters away when the sun appears red at sunrise or sunset.
(d) Gas molecules present in the air.
Which effect is illustrated by the observation that when a beam of sunlight enters a dusty room, then its path becomes visible to us.
The stated effect is known as Tyndall effect.
What is the cause for Hypermetropia ? Show the defective eye and explain how it is converted ?
Hypermetropia : Hypermetropia is also known as hyperopia or farsightedness. Those who deal with hyperopia have difficulty seeing things that are near to them but have a better vision for objects that are far away. For them, the least distance of distinct vision is more than $$25\ cm$$.
This defect of vision arises when the lens or cornea is out of place or insufficiently curved, leading to reduced vision quality.
Depending on how much the focal length has been altered due to the effects of hyperopia, powered convex lenses can be used for the remedy of this defect. The figure shows the defective and the corrected eye.
This defect of vision arises when the lens or cornea is out of place or insufficiently curved, leading to reduced vision quality.
Depending on how much the focal length has been altered due to the effects of hyperopia, powered convex lenses can be used for the remedy of this defect. The figure shows the defective and the corrected eye.
A student claims that because of atmospheric refraction, the sun can be seen after it has set and before the rise, therefore, therefor days become longer than if the earth had no atmosphere.
(a) What does the student mean by saying that the sun can be seen after it has set?
(b) Do you think that the students' conclusion is correct?
When sunlight enters the earth's atmosphere , state which colour of light is scattered the most and which the least .
Sania and Shreya are best friends and study in gradeRecently, Sania has been facing difficulty in reading the blackboard text from the last desk. Shreya is little uncomfortable and wonders why Sania complains sitting on the last desk. On observation, she found that Sania often carries junk food in her lunch. Shreya has started sharing her lunch – full of green vegetables and fruits with her. Sania is now better and has also started taking a 'balanced diet'.
Answer the following questions based on the above situation:
(i) Name the eye defect Sania is suffering from? What are the two possible reasons related to her eye defect?
(ii) What value is displayed by Shreya and Sania?
(iii) How can this value be promoted? Suggest one activity.
(i) From the above discussion, it is clear that Sania is able to see the closer objects but she is not able to see clearly if objects kept at a far distance. This eyes defect is known as Myopia or shortsightedness. In this case focal length of the eyes decreases or the size of eyeball increases so image of the object form ahead of the retina.
(ii) Friendship, concern for each other, appreciation for balanced diet.
(iii) Sharing common things; helping each other in studies.
(ii) Friendship, concern for each other, appreciation for balanced diet.
(iii) Sharing common things; helping each other in studies.
On the rainy day, Ram reached his grandfather's place in village. On the way to house he saw a beautiful rainbow in the sky. In night, he saw lots of twinkling stars in the clear sky. He was very excited to see these beautiful natural phenomena, which he was not able to see in the city, where he lived with his father.
Answer the following questions based on the situation given above.
(i) Do you think that pollution in atmosphere affects the formation of rainbow and twinkling of stars?
(ii) Do you agree with the fact that pollution-free environment will strengthen such natural phenomena in the cities as well?
(iii) What steps can be taken so that the natural phenomena can be enjoyed in cities as well?
How does refraction take place in the atmosphere? Why do stars twinkle but not the planets?
What is a prism?
With the help of a diagram of a prism, indicate its refracting surfaces, refracting edge and base.
A prism is a transparent refracting medium bounded by five plane surfaces inclined at some angle.
The above diagram clearly shows a prism along with it's main components marked.
Figure(a) shows the basic structure of a human eye. Light refracts into the eye through the cornea and is then further redirected by a lens whose shape (and thus ability to focus the light) is controlled by muscles. We can treat the cornea and eye lens as a single effective thin lens.A “normal” eye can
focus parallel light rays from a distant object O to a point on the retina at the back of the eye, where the processing of the visual information begins. As an object is brought close to the eye, however, the muscles must change the shape of the lens so that rays form an inverted real image on the retina. (a) Suppose that for the parallel rays of Figures, the focal length f of the effective thin lens of the eye is $$2.50$$ cm. For an object at distance $$p = 40.0\,cm$$, what focal length f' of the effective lens is required for the object to be seen clearly? (b) Must the eye muscles increase or decrease the radii of curvature of the eye lens to give focal length f'?
Name three refractive defects of vision with the help of diagram. Explain the reasons and correction of these defects.
The three refractive defects of vision are
(i) Myopia
(ii) Hypermetropia
(iii) Presbyopia
(i) Myopia is short-sightedness, the image is formed in front of retina due to the elongation of the eyeball or due to decrease of focal length.
Correction - Using a concave lens.
After using a concave lens, the affected eye can see distant objects
(ii) Hypermetropia is long-sightedness, the image is formed behind the retina due to shortening of the eyeball or due to an increase in the focal length of the lens of the eye.
Correction: Using convex lens.
(c) After using convex lens, the affected eye can see nearby objects
(iii) Presbyopia: It is the defect of an eye in which the power of accommodation of the eye usually decreases with aging. Near point changes as well as the far off objects are also not visible clearly. It is caused due to the weakening of ciliary muscles and the reduced flexibility of eye lens.
Such a defect in which a person suffers from both myopia and hypermetropia is called presbyopia. It is corrected by using bifocal lens.
(i) Myopia
(ii) Hypermetropia
(iii) Presbyopia
(i) Myopia is short-sightedness, the image is formed in front of retina due to the elongation of the eyeball or due to decrease of focal length.
Correction - Using a concave lens.
After using a concave lens, the affected eye can see distant objects
(ii) Hypermetropia is long-sightedness, the image is formed behind the retina due to shortening of the eyeball or due to an increase in the focal length of the lens of the eye.
Correction: Using convex lens.
(c) After using convex lens, the affected eye can see nearby objects
(iii) Presbyopia: It is the defect of an eye in which the power of accommodation of the eye usually decreases with aging. Near point changes as well as the far off objects are also not visible clearly. It is caused due to the weakening of ciliary muscles and the reduced flexibility of eye lens.
Such a defect in which a person suffers from both myopia and hypermetropia is called presbyopia. It is corrected by using bifocal lens.
Draw a diagram to show the formation of image of a distant object by a myopic eye. How can such an eye defect be remedied?
Myopia is a condition in which visual images come to a focus in front of the retina of the eye, resulting in defective vision of distant objects.
What is the cause of rainbow ?
Write a note on twinkling of stars.
The twinkling of stars is due to the atmospheric refraction of the sunlight. The atmospheric refraction occurs in a medium of gradually changing refractive index, i.e. the atmosphere.
Light from stars travel from rarer to denser medium.
Thus, at each layer of the atmosphere, the sunlight bends towards the normal and the apparent position of the star is slightly different from the actual position. So, the star appears to be at S1 instead of S. In addition, this apparent position is not stationary, and it fluctuates. Therefore, the amount of sunlight entering the eyes flickers – the star sometimes appears brighter, and sometimes fainter, which is the twinkling effect.
Previous
Light from stars travel from rarer to denser medium.
Thus, at each layer of the atmosphere, the sunlight bends towards the normal and the apparent position of the star is slightly different from the actual position. So, the star appears to be at S1 instead of S. In addition, this apparent position is not stationary, and it fluctuates. Therefore, the amount of sunlight entering the eyes flickers – the star sometimes appears brighter, and sometimes fainter, which is the twinkling effect.
Previous
Explain different types of vision defects and their rectification.
Explain the formation of rainbow with a neat figure.
A rainbow is a natural spectrum appearing in the sky after a rain shower. It is produced by the dispersion of sunlight by tiny water droplets present in the atmosphere. When a ray of light falls on a water drop, it undergoes refraction and dispersion to form a spectrum. On reaching the opposite side of the drop, this spectrum undergoes total internal reflection (inside the drop). Arriving at the first surface of the drop, each colour is refracted back to air. The angle between the incoming and outgoing rays can be anything between $$0^\circ $$ and about $$42^\circ $$. We observe a bright rainbow when the angle between the incoming and outgoing rays is close to the maximum value of 420. A rainbow is always formed in the direction opposite to that of the Sun
Give the reason for two minute early sunrise.
When a ray of light passes through the atmosphere having layers of different densities and refractive indices, then refraction of light takes place (atmospheric refraction). The actual sunrise takes place when the sun is just above the horizon. When the Sun is just below the horizon, the light rays coming from it, on entering the Earth’s atmosphere suffer atmospheric refraction from a rarer medium to a denser medium. So they move towards the normal after refraction at each layer. Due to continuous refraction of light at each layer of atmosphere, it follows a curved path and reaches the eye of the observer as shown. As a result, we see the Sun two minutes before it rises above the horizon in the morning.
Name the factor on which the scattering of light depends.
Which colour deviates the most due to dispersion?
When light passes through the prism, as the wavelength increases, how does the deviation change? Will it increase or decrease?
Where will be the sun when the rainbow is seen in the east?
Draw a neat diagram showing
(a) neat point of an eye
(b) Hypermetropic eye
(c) correction for Hypermetropic eye.
Pass sunlight through a prism and allow it to fall on a screen. What are the colours seen on the screen?
When sunlight passes through a prism and allow it to fall on a screen there are seven colours seen on the screen. These are Violet, Indigo, Blue, Green, Yellow, Orange, Red.
The beam of white light from a torch is allowed to fall obliquely on the glass prism.
Which are the colours formed on the screen placed behind the prism?
When white light is passed through a glass prism it splits into its spectrum of colours (in order violet, indigo, blue, green, yellow, orange and red) and this process of white light splitting into its constituent colours is termed as dispersion.It happens because different colors light travel with different speed in glass prism hence they split while moving through the prism.
So the beam of white light from a torch when allowed to fall obliquely on the prism splits into seven colors on the screen.The colours formed on the screen are Violet, Indigo, Blue, Green, Yellow, Orange, Red.
What is the defect of vision in the human eye ? State its types and explain in detail.
The inability of ciliary muscles to change the thickness of eye-lens as per requirement and to form the image of an object on retina leads to the defect of vision.
The three main common defects of vision are:
i. Myopia or near-sightedness: It is the inability of eyes to see distant objects clearly. It arises when the eye lens does not become thin as per requirement and as such the image of the object is formed at a distance shorter than retina.
Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.
The three main common defects of vision are:
i. Myopia or near-sightedness: It is the inability of eyes to see distant objects clearly. It arises when the eye lens does not become thin as per requirement and as such the image of the object is formed at a distance shorter than retina.
Myopia can be corrected by using a concave lens of suitable focal length in the spectacles of such a person.
ii. Hypermetropia or far-sightedness: It is the inability of eyes to see nearby objects clearly. It arises when the eye-lens do not become thick as per requirement and as such the image of the object is formed at a distance beyond than retina.
Hypermetropia can be corrected by using a convex lens of suitable focal length in the spectacles of such a person.
Hypermetropia can be corrected by using a convex lens of suitable focal length in the spectacles of such a person.
iii. Presbyopia: This defect of eye arises with aging due to the weakening of ciliary muscles and loss of elasticity of eye-lens. Due to this defect, a person finds it difficult to view nearby as well as distant objects clearly. This defect is corrected by using a bifocal in the spectacles of the person of such a person
During sunset, the sun appears red. What may be the reason?
What may be the reason behind this difference in deviation?
When is the rainbow formed?
A rainbow is caused by sunlight and atmospheric conditions. Light enters a water droplet, slowing down and bending as it goes from air to denser water. The light reflects off the inside of the droplet, separating into its component wavelengths--or colors. When light exits the droplet, it makes a rainbow.
A spectrum is obtained by passing white light from a torch through a glass prism. Draw a figure that represents the experiment.
When sunlight passes through a prism and allow it to fall on a screen there are seven colours seen on the screen. These are Violet, Indigo, Blue, Green, Yellow, Orange, Red.
Which are the occasions when the sunlight has to travel a greater distance through the atmosphere before reaching the eyes of an observer on the earth?
During the sunrise and the sunset, the sun will be near the horizon. This time the sunlight has to travel the longest distance through the atmosphere to reach the eyes of an observer.
A person is not able to see objects beyond $$1.3$$ m. What remedy can you suggest for this defect?
What is the reason for using red light as signal lamps?
Observe the figure
Which are the colours formed on the screen?
When sunlight passes through a prism and allow it to fall on a screen there are seven colours seen on the screen. These are Violet, Indigo, Blue, Green, Yellow, Orange, Red.
The telescope called Chandra X-ray Observatory is placed in the outer space. What is the advantage of placing it there? Explain with reference to the scattering of light in the atmosphere.
Match the columns A, B and C suitably
| A | B | C |
| Light makes tiny particles visible due to scattering | Dispersion | Pictures on a TV screen keep on changing and give an illusion of motion. |
| The effect of seeing an object is retained by the eye for 1/16 of a second. | Tyndal Effect | Formation of rainbow on the horizon |
| Composite light splits up into component colours | Persistence of vision | On foggy morning the path of sunlight becomes clearly visible. |
