Light - Class 8 Physics - Extra Questions

How many times is a ray of light reflected by two plane mirrors placed parallel and facing each other?

if two plane mirrors placed parallel and facing each other the number of ray of light reflected is infinte.


What do you mean by dispersion? Name the different colours of light in the proper sequence in the spectrum of light.

The splitting of light into its component colours is the dispersion of light. The band of coloured components of a light beam is called its spectrum. When white light is dispersed into seven colours by a prism, different colours of light bend through different angles with respect to the incident ray. Out of these seven colours, red light bends the least while violet light bends the most. So the rays of each colour emerge along different paths and become distinct.Hence we get a spectrum of seven different colours in the sequence, Violet, Indigo, Blue, Green, Yellow, Orange and Red. 


Explain how you can take care of your eyes. 

To protect our eyes, the given points should be taken into account :

(1) Visit an eye specialist regularly.

(2) Avoid reading in dim light and very bright light. 

(3) Avoid direct exposure of sun light to the eye. 

(4) Clean your eyes with cold water quickly if dust particles or small insects enter your eye. Do not rub your eyes. 

(5) Maintain a distance of at least 25 cm between the book and your eyes while reading.


Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection?

Regular reflection:

  1. The reflected beams are parallel to each other in regular reflection.
  2. A nicely polished, regular reflective surface produces regular reflection.

Diffused reflection: 

  1. The reflected beams in diffused reflection are not parallel to one another. It happens when a surface is dispersed or uneven.
  2. The surface is rough in diffuse reflection. The laws of reflection are valid. Reflected rays disperse in various directions due to surface imperfections.


Explain how you can take care of your eyes.

Eyes are very sensitive organ of the human body and should be taken care of.

 The following steps should be followed to take care of your eyes:
1. Have a regular checkup with the eye specialist. If advised, use spectacles of proper power after an eye test.
2. Do not rub your eyes. If there is irritation or dust particles enter your eyes, wash them with clean water.
3. Regularly wash your eyes with clean water.
4. Do not read a book too close or too far from the eye as it causes stress.
5. Have a proper diet. Eat food items rich in vitamin A like raw carrots, broccoli and eggs.
6. Avoid overexposure(example looking at sun) or underexposure(example reading in dark) of light to the eye.


Gurmit wanted to perform activity 16.8 using a laser torch. Her teacher advised her not to do so. Can you explain the basis of the teachers advise?

Intensity of the laser torch is very high as compared to the normal torch. Its light is concentrated in a very narrow beam and is highly energetic. It will damage the friend's eye if directly projected into it.


What is meant by 'Dispersion of Light'?

The phenomenon of splitting of white light of prism into its consituent colours is called dispersion of light.


Write a short note on dispersion of light.

The phenomenon of splitting of light into its component colours is called dispersion of light. When white light passes through a glass prism. It spreads out into a band of different colours called spectrum of light. The colours in the spectrum of light are Violet, Indigo, Blue, Green, Yellow, Orange and Red. Formation of a rainbow is an example of dispersion of light in nature. In this case, raindrops are responsible for dispersion of sunlight. 


The human eye forms the image of an object at its .......................

Explanation:-
$$\bullet$$ The human eye forms the image of an object at its Retina.

$$\bullet$$ Retina acts as a screen for the human eye.
Hence answer is $$\textbf{ Ratina}$$


Write any two differences between regular and irregular (diffused) reflection.

Differences between regular and irregular reflection
  Regular Reflection Irregular Reflection
 1 This phenomenon happens with uniform, Polished surface This phenomenon happens with rough surface.
 2 If the incident rays are parallel to each other, then the reflected rays will also be parallel. If the incident rays are parallel to each other, then the reflected rays need not be parallel.


What do you mean by dispersion? Name the different colours of light in the proper sequence in the spectrum of light.

The phenomenon of splitting of white light in component colours is called dispersion.The band of seven colours seen is called spectrum.
Proper sequence of spectrum of light is VIBGYOR (Violet,Indigo,Blue,Green,Yellow,Orange,Red) .


Differentiate between regular and irregular reflection.

Regular Reflection:                                        Irregular Reflection:
(1) It takes place in smooth, well                  (1)It takes place in uneven surface.
polished surface.
(2)Reflection in plane mirror.                        (2)Reflection in ground glass.


Name that part of the eye which is equivalent to the photographic film in a camera.

The retina is that part of the eye that acts as a screen and perceives the image of the objects in front of the eye.


Distinguish between regular and diffused reflection. Does diffused refection mean the failure of the laws of reflection? 

Reflection is what allows us to see thing.Light from a source such a light globe of the sun,strikes the objects and is reflected back to our eyes Reflection from a polished surface is called regular reflection.

In diffused reflection, the reflected rays are not parallel to each other for parallel incident rays. This happens because of the presence of irregular microscopic surfaces not due to the failure of the laws of reflection. Hence, parallel incident rays reflect in different directions. 

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Why can't we see our image in a white sheet of paper through it reflects light?

the surface of the white light sheet reflects the light in different angles 
this multiple reflection scatters the reflecting rays of the image .hence we will be losing the image


Which part of the eye gets affected if someone is suffering from cataract? How is it treated?

When someone is suffering from cataract, his/her crystalline lens (eye lens) is affected as it becomes milky and cloudy.
 It is treated through a cataract surgery.


How is the phenomenon of reflection used in making a kaleidoscope? What are the applications of a kaleidoscope?

A kaleidoscope uses multiple reflection to form beautiful symmetric images. There are three plane mirrors strips inclined at 60 that is fitted in a cylindrical card box. Some coloured pieces of broken bangles or colourful paper bits are placed between the mirrors between two transparent glass sheets, symmetrical images with regular colourful patterns are produced.
Applications of Kaleidoscope:
Since it produces symmetric images, it is useful in studying geometric patterns in mathematics and also a toy.



Write short notes on dispersion of light.

The phenomenon of splitting of light into its component colours is dispersion of light. The band of coloured components of a light beam is called its spectrum. When white light is dispersed into seven colours by a prism, different colours of light bend through different angles with respect to incident ray. Out of these seven colours, red light bend the least while violet light bends the most. So the rays of each colour emerge along different paths and become distinct.Hence we get a spectrum of seven different colours in the sequence, Violet, Indigo, Blue, Green, Yellow, Orange and Red.


If two mirrors are kept at $$60^0 $$to each other, then the number of images formed by them is

Number of images formed by 2 inclined mirrors is given by the relation 

$$\dfrac{360}{\theta} - 1$$ if $$\dfrac{360}{\theta}$$ is even

Number of images formed = $$\dfrac {360}{6} - 1 = 5$$


Mention against each of the following whether regular or diffused reflection win take place when a beam of light strikes. Justify your answer in each case. 
(a) Polished wooden table 
(b) Chalk powder
(c) Cardboard surface 
(d) Marble floor with water spread over it 
(e) Mirror
(f) Piece of paper 

(a) Polished wooden table - regular reflection 
A polished surface is an example of a smooth surface. Hence, reflections from the polished table will be regular. 
(b) Chalk powder - Diffused reflection 
Chalk power spread on a surface Is an example of an irregular surface. It is not smooth. Therefore, diffused reflection will take place
from chalk powder.
(c) Cardboard surface - Diffused reflection 
Cardboard surface is also an example of an irregular surface. Hence, diffused reflection will take place from a cardboard surface. 
(d) Marble floor with water spread over it - Regular reflection 
Marble floor with water spread over it is an example of a regular surface. This is because water makes the marble surface smooth. Hence, regular reflection will take place from this surface.
(e) Mirror - Regular reflection
Mirror has a smooth surface. Therefore, it will give a regular reflection .
(f) Piece of paper - Diffused reflection 
Although a piece of paper may look smooth, but it has many irregularities on its surface. Due to this reason, it will give a diffused reflection.


Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection ? 

$$\textbf{Explanation :}$$

$$\textbf{Regular reflection:}$$ When the reflected rays are parallel after reflection, it is called regular reflection. It takes place in a smooth surface.

$$\textbf{Diffused reflection:}$$ When the reflected rays are not parallel after reflection, it is called irregular or diffused reflection. It takes place through irregular surfaces.

Diffused reflection is not the failure of laws of reflection, it takes place because of irregularities in the reflecting surface.



In a completely dark room, if you hold a mirror, will you see a reflection of yourself in the mirror?

No, we will not be able to see Our reflection in the mirror because there is no source of light in the room.


State two uses of regular reflection.

The reflection from a smooth surface like that of a mirror is called regular reflection.
Uses of Regular reflection - 
(a) We can see our image in a plane mirror due to regular reflection
(b) Formation of images in a periscope is due to regular reflection


State two uses of  : Irregular reflection 

Uses of Irregular reflection -
(a) Any opaque object is visible to us due to irregular reflection
(b) When a movie is projected on the screen by a projector, it can be viewed 
     from different seats in a hall, due to irregular or diffused reflection. 


Match the following: 

List I: Angles between two parallel mirrors
List II: Number of images formed by the two mirrors

When $$\theta=30^0$$, $$n=\frac{360}{30}-1=11$$
When $$\theta=45^0$$,$$n=\frac{360}{45}-1=7$$
When $$\theta=60^0$$, $$n=\frac{360}{60}-1=5$$
When $$\theta=72^0$$, $$n=\frac{360}{72}-1=4$$


Suppose you are in a dark room. Can you see objects in the rooms? Can you see objects outside the room. Explain.

We can see an object only when light rays emitted or reflected from the object reaches our eye. In a dark room, no light reaches our eye. Hence, no objects are visible. Outside the room, all objects are visible because light reflected from their surfaces reaches our eyes.


Fill in the blanks in the following:
(a) A person $$1$$m in front of a plane mirror seems to be __________ m from his image.
(b) If you touch your _______ ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with __________.
(c) The size of the pupil becomes __________ when you see in dim light.
(d) Night birds have _________ cones than rods in their eyes.

(a) A person 11m in front of a plane mirror seems to be $$2\ m$$ from his image.

Distance between mirror and object $$d_1= 1\ m$$
Hence, distance between mirror and image $$d_2= 1\ m$$
Image formed by the plane mirror is behind the mirror. Hence, distance between object and image, $$d = d_1+d_2$$
$$d=1+1=2\ m$$
(b) 
If you touch your $$\textbf{left}$$ ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with $$\textbf{left hand}$$.
Image formed in the plane mirror is laterally inverted. Hence right ear appears as left ear in the image and the left hand appears as right hand in the image.
(c)
The size of the pupil becomes $$\textbf{large}$$ when you see in dim light. 
The eye pupil adjusts itself to capture more light in the dark and increases its size.
(d)
Night birds have $$\textbf{less}$$ cones than rods in their eyes.
Rods are sensitive to dim light and  cones are sensitive to bright light. Since the night birds can see in the night, they have more rods.


State the laws of reflection.

Laws of reflection for light reflecting from a smooth surface are as follows:
1. The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane.
2. Angle of incidence is equal to the angle of reflection.


Describe the construction of a kaleidoscope.

To make a kaleidoscope, get three rectangular mirror strips. Join them together to form a prism. Fix them in a circular cardboard tube or tube of a thick chart paper. Make sure that the tube is slightly longer than the mirror strips. Close one end of the tube by a cardboard disc having a hole in the centre, through which you can see At the other end, touching the mirrors, fix a circular plane glass plate. Place on this glass plate several small pieces of coloured glass. Close this end of the tube by a ground glass plate. Allow enough space for the colour
pieces to move around. Your kaleidoscope is ready. 
When you peep through the hole, you will be able to see a variety of patterns in the tube. Interesting feature of a kaleidoscope is that you will never see the same pattern again. 


How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by $$40$$cm?

Object placed between two parallel plane mirrors form infinite images. Hence, infinite images of the candle will be formed.


Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case.
(a) Polished wooden table
(b) Chalk powder
(c) Cardboard surface
(d) Marble floor with water
(e)Mirror
(f) Piece of paper

Regular reflection occurs at perfectly smooth surfaces and diffused reflection occurs at rough surfaces. 
(a) Polished wooden table: Regular reflection since surface becomes smooth as it is unpolished.
(b) Chalk Powder: Diffused reflection since surface of chalk powder is very rough.
(c) Cardboard surface: Diffused reflection since surface of chalk powder is rough.
(d) Marble floor with water:  Diffused reflection since surface of water is not even.
(e) Mirror: Regular reflection since surface of mirror is smooth.
(f) Piece of paper: Diffused reflection since surface of paper is rough.


What is the angle of incidence of a ray if the reflected ray is at an angle of $$90^\circ$$ to the incident ray?

By the laws of reflection, angle of incidence ($$i$$) is equal to the angle of reflection ($$r$$). 
i.e. $$i=r............(i)$$
Also, they are on the opposite sides of the normal. Since, they are in the same plane, angle between them $$\theta = i+r...........(ii)$$
From $$(i)\ \&\ (ii),$$
$$\theta = 2i$$
It is given $$\theta = 90^\circ$$
$$\implies i=45^\circ$$


A ray of white light shows no dispersion on emerging from a glass plate (slab). Is there no dispersion inside the glass plate as well?

Dispersion phenomena take place in a glass prism. But in glass slab light disperses into different colors on entering the glass slab i.e. dispersion takes place inside a glass slab also. But when it emerges from its other parallel face, the different colors again combine to form white light.


Fill in the blanks
The splitting of white light into different colours (VIBGYOR) is called ___________.

The splitting of white light into different colours (VIBGYOR) is called Dispersion.


An object is placed near two perpendicular plane mirrors shown in the figure. How many images will be formed?
512294_1377a91a268e45a2ad88a7423c424b16.png

Hence according to formula
number of images $$= \dfrac{360}{\theta}-1$$
Here $$\theta=90^{0}$$
Hence number of images $$=\dfrac{360}{90}-1   =3$$

899898_512294_ans_52e4224cb6874e9ebf4597427370b7ba.png


If Raman fixed two mirrors at an angle of $$60 ^{\circ}$$ to get as many number of images, could you find out the exact number of images produced?

Since we know that number of images formed by plane mirrors arranged at an angle $$\theta$$ to each other are
$$N=\dfrac{360}{\theta}-1$$
So according to question
$$\theta=60^{0}$$
Hence $$N=\dfrac{360}{60}-1 =5$$
$$N=5$$

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Identify the mistakes and correct them.
The beautiful pattern that we obtain in a kaleidoscope is because of refraction.

The beautiful pattern that we obtain in a kaleidoscope is because of reflection.
The kaleidoscope is an optical instrument that works on the principle of multiple reflections. It consists of two or more reflecting surfaces tilted at some angles.


A light ray is incident on a plane mirror with $$30^o$$ angle of incidence. What will be the angle of reflection?

Given,
Angle of incidence, $$i=30^\circ$$
According to laws of reflection, the angle of incidence $$i$$ is equal to the angle of reflection $$r$$.
$$i=r$$
$$\implies r=30^\circ$$


The working principle of a periscope is _________.

The working principle of a periscope is the law of refletion. 


Objects present in the dark room are not visible. But when the light is switched on, everything present in the room becomes visible.Why does this happen?

An object is visible to our eye when the light rays reflected by the object are able to enter our eye. Hence we cannot see an object in dark room whereas we can see them when we switch on the light.


What is 'dispersion of white light'? State its cause. Draw a ray diagram to show the dispersion of white light by a glass prism.

The splitting of white light into its component colours is called dispersion.

All the colours travel in the same speed when they travel in vacuum. But when they hit a medium, such as a glass prism, the colours bend or refract at different angles, and therefore the speed changes. Thus they emerge in different, distinct directions which seem like a spectrum to us. Red colour bends the least and violet deviates the most . In short, we can say that the cause of this splitting of white light into its component colours is due to the different speeds of the colours in the white light when it hits a particular medium.

The diagram for dispersion of white light by a glass prism is shown.

658666_624552_ans_fde2402a93144eda89e0a327dcd8b461.png


How do you verify the 1st law of reflection of light with an experiment?

We can use a laser light and make it incident on a plane mirror and we can observe the path of reflected ray so from that we can observe that incident ray,normal and reflected ray at point of incidence all lie in same plane .


Classify the objects or materials given below as opaque, transparent or translucent and luminous or non-luminous.
Air, water, a piece of rock, a sheet of aluminium, a mirror, a wooden board, a sheet of polythene, a CD, smoke, a sheet of plane glass, fog, a piece of red hot iron, an umbrella, a lighted fluorescent tube, a wall, a sheet of carbon paper, the fame of a gas burner, a sheet of cardboard, a lighted torch, a sheet of cellophane, a wire mesh, kerosene stove, sun, firefly, moon.


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Why is the moon not considered as a luminous body?

Moon is a non-luminous body because it shines by reflecting the sunlight falling on it. Moon does not have its own light.


State difference between a luminous and a non-luminous body.

The bodies which emit light are called luminous bodies. Example: Sun, stars, burning candle etc. The bodies which does not emit light are called non-luminous bodies. Example: Moon, Earth, blackboard.


A plane mirror makes an angle of $$30^o$$ with the horizontal. If a vertical ray strikes the mirror. Find the angle between the mirror and the reflected ray?

Hint: Use the concept of the law of reflection.

Explanation:
Step 1: Draw the ray diagram:



Step 2: Finding the angle of incidence and reflection:

Since angle between horizontal and mirror is $$30^o$$
Now taking vertical incident ray with mirror is $$90-30=60^o$$
Now angle of incidence $$i=90-60=30^o$$
By law of reflection,
$$ \textbf {Angle of incidence=Angle of reflection}$$
Now angle of reflection $$r=30^o$$

Step 3: Conclusion:
Angle made by a reflected ray with mirror=$$90-r=90-30=60^o$$

Hence $$60^o$$ is correct answer.


A ray of light is incident at an angle of $$35^0$$ on a plane mirror. What is angle $$\theta$$?
1063046_ff483eec86fa42d3b361061efd36c23f.png

Angle of incidence   $$i = 90-35 = 55^o$$
From laws of reflection,  angle of reflection  $$r = i = 55^o$$
Thus   $$\theta = 180^o - i-r-35^o$$
$$\implies \ \theta = 180^o-55^o-55^o-35^o = 35^o$$


Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane.

This activity can be performed using plane mirror. For that take a sheet of paper with hole in its center.

Two things needed to take care that light in room is not too much bright and sheet is normal to the table.

Put another sheet on table such that it is in contact with vertical mirror and normal line is drawn from the mirror to second sheet. Now, turn on the torch such that light is passing through small hole and reached to the mirror and light falls on normal at mirror at bottom.

After ray passes through hole and incident on mirror, there will be reflection in certain direction. From the observation of rays, one can clearly see that normal to the mirror, incident ray and reflected ray all are on the sheet placed on the table. Which suggests that incident ray, reflected ray and normal to the surface at the point of incidence all 3 lies on the same plane.



Find no of images & draw all the images termed.
1023747_c05597a7118b445db2bc80863055a08c.png


The angle between the two plane mirror=$${120^0}$$

calculate, $$\dfrac{{360}}{\theta}$$ 
Since, $$\dfrac{{360}}{{120}} = 3$$, it is an odd number
Hence, no of image formed is given by =$$\dfrac{360}{\theta}=\dfrac{{360}}{{120}} = 3$$

Therefore, three image are formed between the plane mirror.

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Make a model of kaleidoscope.

A kaleidoscope is composed of a hollow tube. Three mirrors are placed in the form of a triangular tube and their reflecting surfaces face each other. One end of the tube is covered with a transparent sheet and another end is covered with an opaque sheet. There is an eyehole in the opaque sheet. Bits of glass are filled inside the tube. When a kaleidoscope is turned, we get to see various patterns in it. These patterns are formed because of multiple reflections.




State the cause of dispersion of white light by a glass prism. How did Newton, using two identical glass prisms, show that white light is made of seven colors? Draw a ray diagram to show the path of a narrow beam of white light, through a combination of two identical primes arranged together in inverted position with respect to each other, when it is allowed to fall obliquely on one of the faces of the first prism of the combination.

Newton's experiment with two identical prisms: . When a beam of white light is passed through a glass prism, a band of seven colors is formed on a white screen. ... Newton showed that the seven colored lights of the spectrum can be recombined to give back white light

The cause of dispersion is the change in speed of light with wavelength .when white light enters the first surface of a Prism ,light of different colours due to their different speeds in glasses gets deviated towards the base of the prism through different angles that is the Dispersion of white light into its constituent colours takes place at the first surface of prism.

Newton's experiment -

Newton allowed white light from the sun to enter a dark room with through a small aperture in a window and placed a glass prism in the path of white light .the light coming out of the prism was received on a white screen .on the screen a coloured patch like a rainbow was found which was termed as spectrum .starting from the side of the base of the prism the colours of the spectrum on the screen in the following order violet Indigo blue green yellow orange and red the order of colouring



What type of reflection will take place from a polished surface?

Regular reflection takes place from a smooth or a regular surface. In regular reflection, all reflected rays are parallel to each other for parallel incident rays. Irregular or diffused reflection takes place from an irregular surface.


What is the dispersion of light? Name seven colours of while light.

Visible light, also known as white light, consists of a collection of component colors. These colors are often observed as light passes through a triangular prism. Upon passage through the prism, the white light is separated into its component colors - red, orange, yellow, green, blue and violet. The separation of visible light into its different colors is known as dispersion.



When is a person said to have developed a cataract in his eye? How is the vision of a person having a cataract restored?

A person is said to have developed a cataract when the eye lens becomes progressively cloudy resulting in blurred vision. The vision of a person having a cataract can be restored after getting surgery done on the eye having a cataract.


What is the difference between regular reflection and diffuse reflection of light ? Name one object which can produce regular reflection of light and another which produces diffuse reflection of light. 

Reflection from a smooth surface like that of a mirror is called regular reflection. Images are formed by regular reflection. Parallel incident rays are reflected as parallel rays due to the laws of reflection.

On the other hand, when all the parallel rays reflected from a rough or irregular surface are not parallel, the reflection is known as diffused or irregular reflection. The diffused reflection is not due to the failure of the laws of reflection. It is caused by the irregularities in the reflecting surface, like that of a cardboard. 

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Fill in the blanks :
The reflection of light from a wooden surface is ..................reflection.

The reflection of light from a wooden surface is diffused reflection.


What is cataract?

A cataract is the clouding of the crystalline lens of the eye which often affects the clarity of vision. Generally, it’s a function of maturity, or aging, as a clear lens in the eye becomes opaque and foggy. Just about everyone will develop cataracts if they live long enough due to normal exposure to UV light from the sun.


Draw a scientifically correct labelled diagram of a human eye and answer the questions based on it :
Name the type of lens in the human eye.

The human eye works on refraction of light through a natural convex lens made of transparent living material & enables us to see things around us.
So, eye lens is the convex lens.

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Whart is the difference between regular reflection of light and diffuse reflection of light? What type of reflection of light takes place from 
(a) a cinema screen (b) a plane mirror (c) a cardboard (d) still water surface of a lake

In a regular reflection, a parallel beam of incident light is reflected as a parallel beam in one direction, while in diffusion reflection, a parallel beam of incident is reflected in different direction.
(a) Regular reflection
(b) Regular reflection 
(c) Diffuse reflection
(d) Regular reflection


(a) A wall reflects light and a mirror also reflects light. What difference is there in the way they reflect light ?
(b) Which type of reflection of light leads to the formation of images ?

(a) A wall has a rough surface, so the reflection by a wall is a diffuse reflection. A parallel beam of light incident on it is reflected in different directions.
A mirror surface is smooth, so the reflection by the mirror is a regular reflection. A parallel beam of light incident on it, gets scattered by making reflected rays in different directions.
(b) Regular reflection


State the correct sequence $$(1-7)$$ of colours in the spectrum formed by the prisms A and B, shown in the image.
1789573_50bae0819a194ba2afd76b761a2db7b7.png

In figure A and Figure B

Figure A       Color          Figure B
$$1$$Red$$7$$
$$2$$Orange$$6$$
$$3$$Yellow$$5$$
$$4$$Green$$4$$
$$5$$Blue$$3$$
$$6$$Indigo$$2$$
$$7$$Violet$$1$$


Name a natural phenomenon which is caused by the dispersion of sunlight in the sky.

Rainbow.
The formation of a rainbow is a natural phenomenon which is caused by the dispersion of sunlight in the sky. The sunlight splits into 7 colors: violet, indigo, blue, green, yellow, orange, and red.


Draw a labelled diagram of human eye and explain the image of formation. 

the figure below shows the labelled diagram of the human eye . the main parts of eye are :-
Cornea , Iris , Pupil , Ciliary muscles , Eye lens , retina and optical nerves which are labelled in the diagram below . 

Image Formation : The light rays coming from object enter through of eye , pass through the pupil of the eye and fall on the eye lens . the eye lens is a convex lens , so it produces real and inverted image of object. 
The optical nerve conveys the image formed on retina to brain and we get sensation of vision.
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Among animals, the predators (like lions) have their eyes facing forward at the front of their heads, whereas the prey (like rabbit) usually have eyes at the sides of their head. Why is this so?

The predators (like lions) have their eyes facing forward at the front of their heads so as to detect and focus on the prey, whereas the prey (like rabbit) usually have eyes at the sides of their head so that they can see their enemies (predators) in a very large area around them and try to escape from them.


What did Newton demonstrate by his experiments with the prism?

Newton demonstrated by his experiments with the prisms that white light consists of a mixture of seven colours.


The diagram above shows the structure of the eye. Choose the words from the list below to label the parts A , B , C and D (ciliary muscles , pupil , retina , cornea, iris , optic nerve). 
1771633_c4e464f24c004121815032e2354c1fd9.png

The figure below shows the labeled diagram. 

A. Retina: It is a transparent, jelly-like mass located behind the lens. The images are formed on this. So, it acts as a screen.

B. Cornea: It helps to maintain the shape of the anterior chamber of the eyeball.

C. Iris: A part of the retina that is directly opposite to the pupil and contains only cone cells. It is responsible for good visual acuity (good resolution). 

D. Optic nerve: The optic nerve is responsible for vision. Each nerve contains approx. one million fibers transmitting information from the rod and cone cells of the retina.  
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Boojho made light from a laser torch to fall on a prism. Will he be able to observe a band of seven colours? Explain with a reason.

No, he will not be able observe the band of seven colours because laser light is made up of only a single colour.

he would have observed band of seven colours if white light was made to fall on prism , because white light is actually made up of seven colors.



Fill in the blanks:
We are able to see the objects around us due to ________ reflection.

We are able to see the objects around us due to irregular reflection.Because due to irregular reflection  light rays go in any random direction so an object is able to send light rays after reflection in all direction and hence the objects are visible to us.


Differentiate between luminous and non luminous body. Give two example of each.

Luminous Bodies:
The bodies which have light their own e.g sun, star, bulb ,candle,oil lamp, torch, lantern.
Non luminous Bodies:
The bodies which do not have their own light e.g moon, chair ,table, when light falls on them , they become visible.


Is the moon a luminous object?

Moon is not a luminous body, because It has no lights of its own.


Describe an experiment to verify the laws of reflection of light. 

Laws of reflection.  
(i) The incident ray, the reflected ray and the normal at the point of incidence, lie in the same plane. 
(ii) The angle of Incidence and angle of reflection are equal I.e. $$\angle i = \angle r $$. 

Verification 
Take a wooden drawing board and fix a white sheet of paper on it. In the middle of paper draw a straight line KK\ Mark a point B on it. Draw a perpendicular BN. Place a mirror XX' on line KK' such that the polished side of the mirror is along the line. Hold the mirror in the mirror holder.
Fix two steel pins P and Q on the straight line AB at least 10 cm apart. Look for the images of the pins P and Q and fix two pins P a such that P', Q', and images of P and Q are all in the same straight line. Remove the pins and draw small circles around the pinpricks. 
Remove the mirror also. Join P'Q' and produce the straight line to meet at B. Measure $$\angle ABN$$ = i and $$\angle CBN$$ = r. 
It is found that $$\angle i = \angle r$$. This proves that the Angle of Incidence is equal to the Angle of Reflection. 
As the incident ray, the reflected ray and the normal lie in the plane of the paper, therefore, they lie in the same plane. 

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Write down the names of parts of the eye in the blank spaces shown in the given figure.
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(1) Cilliary muscle
(2) Iris
(3) Lens
(4) Cornea
(5) Retina
(6) Optic Nerve


 Name the part of the eye which gives colour to the eyes.

Iris gives colour to the eyes.
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What kind of lens is there in our eyes? Where does  it form the image of an object?

We have convex lens in our eyes. It forms the object on the retina by focusing the light at the backside of the eye.


What do you understand by regular and irregular reflection?

When all reflected rays are parallel to each other, the reflection is called regular reflection. Clear image is formed in case of regular reflection. When reflected rays are not parallel to each other, the reflection is called irregular reflection. Diffused image is formed in case of irregular reflection.


With the help of diagrams , explain the difference between regular and irregular reflection. 

When a ray or beam of light falls on a smooth and polished surface an irregular reflection is observed when the light falls on a rough surface . As the surface is uneven ,light rays from several points gets reflected in different directions ,producing irregular reflections.
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What Is irregular reflection? Give an example. 

Irregular reflection - When a beam of light falls on such a surface that Is not perfectly smooth and polished such as a wall, wood, paper, etc. the different portions of the surface reflect light in different directions. Such a reflection of light in different directions. Such a reflection of light from an uneven surface is called the irregular or diffused reflection. For example, the reflection of light from the surface of a flowing stream.
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Define the term dispersion of light . 

The phenomenon of splitting of white light by a prism into its constituent colors is known as dispersion of light.


One can sense colours only in bright light.

The cells present on the retain and responsible for colour vision are known as cone cells. These cells become active only under bright light and remain inactive under dark. Thus, we are able sense only in bright light.


What is dispersion of light?

The splitting of light into its various components (i.e., 7 colours) is called dispersion of light.


How will you explain the statement 'we cannot see the objects in a dark room?

Light enables us to see objects around us. We see an object when light falls on it and gets reflected from its surface and enters our eyes. In a dark room, there is no source of light. Thus, no light falls on the surface of objects and hence we do not see them. This is why we cannot see the objects in a dark room.


Study the following incident.
Swara and Yash were looking in a water filled vessel. They could see their images clearly in the still water. At that instant, Yash threw a stone in the water. Now their images were blurred. Swara could not understand the reason for the blurring of the images. 
Explain the reason for blurring of the images to swara by answering the following questions.
Which types of reflection of light can you notice from this?

Initially, the water was still and its surface was smooth. Thus, regular type of reflection was taking place from its surface i.e. all the light rays coming from Swara's body or Yash's body reflected in the same direction on hitting the water surface and reached our eyes. Because of this, we saw a clear image. 
But when Yash threw a stone in the water, the water surface became rough due to which irregular type of reflection of light occurred on its surface i.e., all the light rays coming from Swara's body or Yash's body reflected in different directions on hitting the water surface. Thus, only some part of the reflected rays reached our eyes. Because of this, we see a blurred image.


Explain in brief the working and structure of human eye.

In our eyes there is convex lens located which is formed by flexible muscles. Due to this lens, image of object formed at retina, and we can see things. Our eyes are round in shape. the outermost conversing of eye is white in front of it, transparent part cornea is situated, behind cornea dark coloured structure made of muscles is located, it is called iris. There is a small hole in iris called Pupil. Its size is controlled by the iris. In this case of more light the aperture of pupil reduces and in low light it increases. At the back of the pupil is eye lens which is located at its place with the help of muscles. The part between cornea and lens is filled with transparent fluid which is called eye fluid or Aqueous humour.
Through lens inverted image is formed on retina. Retina is a light absorber transparent membrane which has many light sensitive nerves, these are connected to brain. When these nerves send the message of the formation of image on retina to the brain, then brain converts inverted image into erect image and we see the object. The transparent fluid fill between lens and retina is called vitreous humour or fluid.
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Draw a neat labeled diagram of human eye and explain the working of each part of it.

Working of Human eye human eye consists of various parts which help us in seeing the objects, the function of various parts are :
(a) Cornea: It is the transparent membrane which refracts the light entering our eye.
(b) Iris: Iris controls the size of pupil.
(c) Pupil: It allows the light to enter our eye to pass through it.
(d) Lens: Adjusts the focal length of the eye to see the objects at different places.
(e) Ciliary muscles: Helps in changing the focal length of the lens.
(f) Retina: It is the screen of the eye on which image is formed. It consists of rods and cones,
(g) Optic nerve: It carries the electrical signals from retina to brain.
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Study the following incident.
Swara and Yash were looking in a water filled vessel. They could see their images clearly in the still water. At that instant, Yash threw a stone in the water. Now their images were blurred. Swara could not understand the reason for the blurring of the images. 
Explain the reason for blurring of the images to swara by answering the following questions.
Are laws of reflection followed in these types of reflection ?

Yes, laws of reflection are followed in both the types of reflection.


What is called regular and irregular reflection of light?

Regular reflection of light occurs when a parallel beam of light incident on a smooth and polished surface remains parallel after reflection in a specific direction.
When a parallel beam of light is incident on a rough or irregular surface, the beam does not remain parallel but spreads over a wide region after reflection. Such a reflection of light is called irregular reflection.


What is a blind spot ?

Blindspot is a small area of the retina insensitive to light where the optic nerve leaves the eye .
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What is the dispersion of light ? Which are the colours of the spectrum obtained from the dispersion through a glass prism ?

The phenomenon of splitting of white light into its constituent colours on passing through a glass prism is called dispersion of light.Violet, indigo, blue, green, yellow, orange and red colours are obtained from the dispersion of light through glass prism
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What are the functions of Iris and Pupil in human Eye ?

The iris controls the amount of light entering the eyes. The Iris automatically adjusts the size of the pupil according to the intensity of light received by the eye. If the amount of light received by the eye is larger, then the iris contracts the pupil and reduces the amount of light entering the eye. 

On the other hand if the amount of light received by the eye is small the iris expands the pupil so that more light may enter into the eyes.


Which phenomenon is responsible for Dispersion of Light.

Refraction is the correct reason responsible for the dispersion of light. Dispersion of light is defined as the phenomena of splitting up of composite light into its constituent colours.
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What is reflection of light ? State two laws of reflection.

When a ray of light approaches a smooth polished surface and the light ray bounces back, it is called the reflection of light.
Laws of reflection are: 
(i) The incident ray, the reflected ray and the normal ray at the point of incidence, lie in the same plane. 
(ii) The angle of incidence is equal to the angle of reflection.

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Fill in the blanks:
A ______ splits a beam of sunlight into seven colors.

A prism. splits a beam of sunlight into seven colours.
The splitting of white light into its component colours is called dispersion.


Moon is a ______ body.

Moon is a $$\text{non-luminous }$$ body.
Moon is a non-luminous body because it shines by reflecting the sunlight falling on it. It does not have its own light.



Does the flame of a gas stove emit light?

Any burning thing emit light. The intensity of light may not be same. A !lame of a gas stove emit light. 


What do we call a body that shines on its own ?

The body that shines on its own is called a luminous body. 


What is dispersion of light ? What is the cause of dispersion ? Draw a diagram to show the dispersion of white light by a glass prism.

Dispersion of Light: The phenomenon of splitting white light into seven colours when it passes through a glass prism is called dispersion of light. 
Cause of Dispersion: When white light passes through a glass prism, its constituent colours (red, orange, yellow, green, blue, indigo, violet) travel with different speeds in the prism because refractive index is color dependent. This causes the dispersion of light.

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Why is the colour of clear sky blue ? Explain.

When sunlight enters the earth’s atmosphere, the atoms or molecules of the gases present in the atmosphere scatter this light. Since the wavelength of red colour is larger than the wavelengths of other colours in sunlight, so red colour is scattered least. Violet colour is scattered the most followed by blue, green, yellow, orange and red colours respectively. Our eye is more sensitive to the blue light than the violet light. Therefore, scattered light in the sky contains blue colour in plenty and hence the clear sky appears blue.
Note : Sky appears greyish over cities having industrial units. The smoke and dust particles in the atmosphere over such cities scatter red, orange and yellow colours more than other colours of small wavelengths. Hence, the sky appears greyish.


Name the type of lens in human eye.

Human eye has a convex lens.
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Non-luminous objects are those that do not emit their own ___________.

Light.
Objects like the moon that do not give out or emit light of their own are Non- luminous objects. Moon is an example of a non-luminous object as we can see the moon because it reflects light from the sun. Other examples of Non-luminous bodies are pen, pencil, chair, wood, etc.


An incident ray strikes a plane mirror at an angle of $$20^o$$ with respect to horizontal surface as shown. What is the change in the angle of reflection, if the incident ray moves towards normal by $$1^o$$?
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Now angle of distance $$i=90^{0}-20^{0}=70^{0}$$
Angle of reflection is $$r=i=70^{0}$$                (from laws of reflection)
If angle of incidence decreases by $$1^{0}$$ so angle of reflection will also decrease by $$1^{0}$$.

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An object is kept in front of three mutually perpendicular mirrors. Then the total number of images is.

The three mutually perpendicular mirrors form an image in every quadrant of the $$3$$ dimensional space with an intersecting point of the $$3$$  mirrors as the centre of the space.
So let's demonstrate this by denoting anything that is in a quadrant space of $$3$$-dimensional space by $$xyz \ x'y'z'$$, where $$x,y,z$$ are positive axes and $$x',y',z'$$ are negative axes. so in this way, our given object is in $$xyz$$ space($$xyz$$ quadrant where all $$x,y,z$$ are positive).
Now the 3 planes of the $$xyz$$ quadrant,i.e $$xy$$ plane, $$yz$$ plane,and $$zx$$ plane are mirrors and so they form corresponding mirror images in $$xyz', yzx'$$ , and $$zxy'$$ quadrants. and also considering the mirror images of the $$3$$ images formed directly from the object, we get an image in every quadrant of the $$3$$-dimensional space except the quadrant containing the object itself,
So the total images formed are $$7.$$

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State and explain the laws of reflection of light at a plane surface (like a plane mirror), with the help of a labelled ray-diagram. Mark the angles of incidence and reflection clearly on the diagram. If the angle of reflection is $$45.5^{\circ}$$, what will be the angle of incidence ?

Laws of reflection of light:
First law of reflection: According to the first law of reflection, the incidence ray, the reflected ray and normal (at the point of incidence), all lie in the same place. For e.g., in the figure, the incidence ray AO, the reflected ray OP and the normal ON, all lie in the same plane, the plane of paper.
Second law of reflection: According to the second law of reflection, the angle of reflection is always equal to the angle of incidence. For e.g., if we measure the angle of reflection NOB in the figure, we will find that it is exactly equal to the angle of incidence AON.
If the angle of reflection is $$45.5^{\circ}$$, the angle of incidence will also be $$45.5^{\circ}$$ in accordance with the second law of reflection.
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Draw diagrams to show the regular reflection of light and diffuse reflection of light. 


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The diagram in fig. shows an incident ray AO and the normal ON on a plane mirror. Draw the reflected ray. State the law you use to draw the direction of the reflected ray. 
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The Law of reflection of light is used to draw the direction of the reflected ray. 
This law states that the angle of incidence is equal to the angle of reflection. $$\angle i = \angle r$$ 

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Why do the windows appear less bright than the white walls of a house in daylight?

When light rays fall on the wall of a building, they scatter everywhere leading to a brighter image of the wall. But when light rays fall on a glass window, they suffer regular reflection due to the smoothness of the surface of the glass windows. Thus, we get reflected light rays in a particular direction. Therefore, the windows will appear less bright.


Why can the text on a page of a book be read through tissue paper only if the paper is placed directly on the page?

When the tissue paper is placed directly on the page the illumination will not be uniform everywhere. The parts of the paper which are in contact with the texts will be less illuminated. Therefore, those will be comparatively dark as compared to the other parts. Thus, it will be easy to read the text.


Class 8 Physics Extra Questions