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CBSE Questions for Class 10 Physics Light Reflection And Refraction Quiz 12 - MCQExams.com
CBSE
Class 10 Physics
Light Reflection And Refraction
Quiz 12
The radius of curvature of a concave mirror is $$40\ cm$$ and the size of the real image is twice as that of an object, then the object distance is:
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$$60\ cm$$
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$$20\ cm$$
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$$40\ cm$$
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$$30\ cm$$
Explanation
Given, Concave mirror
Radius of curvature, $$R = -40\ cm $$
Magnification, $$m = -2 $$ (for real image $$m$$ is negative)
Focal length, $$f = \dfrac{R}{2} = \dfrac{-40}{2} = -20\ cm $$
We know, $$m = \dfrac{-v}{u} $$
$$ -2 = \dfrac{-v}{u}$$
$$ \Rightarrow u = \dfrac{v}{2} $$
$$\Rightarrow v = 2u $$
Using mirror formula,
$$ \dfrac{1}{f} = \dfrac{1}{v} + \dfrac{1}{u} $$
$$\Rightarrow \dfrac{1}{-20} = \dfrac{1}{2u} + \dfrac{1}{u} = \dfrac{3}{2u} $$
$$\Rightarrow 2u = 3 \times (-20) $$
$$u = \dfrac{-60}{2} = -30\ cm $$
$$u = -30\ cm $$
The object is $$30\ cm $$ from the mirror.
The ratio of thickness of plates of two transparent medium $$A$$ and $$B$$ is $$6:4$$. If the takes equal time in passing through them, then refractive index of $$B$$ with respect to $$A$$ will be
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$$1.4$$
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$$1.5$$
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$$1.75$$
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$$1.33$$
Explanation
Time taken by light to travel distance $$x$$ through a medium of refractive index $$\mu $$ is
$$t=\dfrac {\mu x}{c}\Rightarrow \dfrac {\mu_B}{\mu_A}=\dfrac {x_A}{x_B}=\dfrac {6}{4}\Rightarrow _A\mu_B =\dfrac {3}{2}=1.5$$
The time required to pass the light through a glass slab of $$2\ mm$$ thick is $$(\mu_{glass}=1.5)$$
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$$10^{-5}s$$
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$$10^{-11}s$$
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$$10^{-9}s$$
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$$10^{-13}s$$
Explanation
As we know,
$$t=\dfrac {\mu x}{c}=\dfrac {1.5\times 2\times 10^{-3}}{3\times 10^8}=10^{-11} sec$$
An object $$1\ cm$$ tall is placed $$4\ cm$$ infront of a mirror. In order to produce an upright image of $$3\ cm$$ height one needs a
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Convex mirror of radius of curvature $$12\ cm$$
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Concave mirror of radius of curvature $$12\ cm$$
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Concave mirror of radius of curvature $$4\ cm$$
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Plane mirror of height $$12\ cm$$
Explanation
Erect and enlarged image can be produced by a concave mirror.
From the definition of magnification we get,
$$m=\dfrac{\text{Height of the image}}{\text{Height of the object}}=+\dfrac{3}{1}=+3$$
We also know that $$m=-\dfrac{v}{u}=+3$$ $$\Rightarrow v=-3u=-3\times (-4)=12\ cm$$
Using mirror formula we get $$\dfrac{1}{v}+\dfrac{1}{u}=\dfrac{1}{f}$$
$$\therefore \dfrac{1}{12}+\dfrac{1}{-4}=\dfrac{1}{f}$$
$$\Rightarrow \dfrac{1-3}{12}=\dfrac{1}{f}$$
$$\Rightarrow f=-6\ cm$$
The radius of curvature of the mirror is $$R=2f=2\times 6 = 12\ cm$$, and it is a concave mirror as the focal length is -ve.
Which of the following ray diagram show physically possible refraction
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(i)
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(ii)
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(iii)
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None of these
Explanation
When light ray goes from denser to rarer medium (i.e. more $$\mu$$ to less $$\mu$$) it deviates away from the normal
On the other hand, if light ray goes from rarer to denser medium (i.e. less $$\mu$$ to more $$\mu$$) it bends towards the normal.
It is followed only in option A.
In option B, the refracted ray is on the same side of the normal as the incident ray, which never happens in case of any refraction. So it is incorrect.
What is the time taken $$(in\ s)$$ to cross a glass of thickness $$4\ mm$$ and $$\mu=3$$ by light?
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$$4\times 10^{-11}\ s$$
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$$2\times 10^{-11}\ s$$
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$$16\times 10^{-11}\ s$$
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$$8\times 10^{-10}\ s$$
Explanation
T
hickness of the glass, $$d = 4\ mm = 4 \times 10^{-3}\ m $$
Refractive index of the glass $$\mu = 3 $$
We know
$$\mu = \dfrac{\text{Speed of light in vacuum}}{\text{Speed of light in glass}} = \dfrac{c}{v} $$
$$3 = \dfrac{3 \times 10^{8}}{v} $$
$$\Rightarrow v = 1 \times 10^{8}\ m/s $$
$$\text{Speed} = \dfrac{\text{distance}}{\text{time}} $$
$$\Rightarrow \text{time} = \dfrac{\text{distance}}{\text{speed}} = \dfrac{4 \times 10^{-3}\ m}{1 \times 10^{8}\ m/s} $$
$$\text{time}, t = 4 \times 10^{-11}\ s $$
A convex mirror has a focal length $$f.$$ A real object is placed at a distance $$f$$ in front of it from the pole produces an image at
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Infinity
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$$f$$
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$$f/2$$
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$$2f$$
Explanation
Here focal length $$=f$$ and $$u=-f$$
On putting these values in $$\dfrac{1}{f}=\dfrac{1}{u}+\dfrac{1}{v}$$
$$\Rightarrow \dfrac{1}{f}=-\dfrac{1}{f}+\dfrac{1}{v}\Rightarrow v=\dfrac{f}{2}$$
Match List $$I$$ with List $$II$$ and select the correct answer using the codes given below the lists:
List $$I$$
List $$II$$
(Position of the object)
(Magnification)
(I) An object is placed at focus before a convex mirror
(A) Magnification is $$- \infty$$
(II)An object is placed at centre or curvature before a concave mirror
(B) Magnification is $$0.5$$
(III) An object is placed at focus before a concave mirror
(C) Magnification is $$+1$$
(IV) An object is placed at centre or curvature before a convex mirror
(D) Magnification is $$-1$$
(E) Magnification is $$0.33$$
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$$I-B, II-D, III-A, IV-E$$
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$$I-A, II-D, III-C, IV-B$$
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$$I-C, II-B, III-A, IV-E$$
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$$I-B, II-E, III-D, IV-C$$
A medium shows the relation between the angle of incidence $$i$$ and the angle of refraction $$r$$. If speed of light in the medium is $$nc$$ then value of $$n$$ is
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$$1.5$$
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$$2$$
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$$2^{-1}$$
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$$3^{-1/2}$$
Explanation
Let $$\mu$$ be the refractive index of the medium.
From Snell's law: $$\mu=\dfrac{sin\ i}{sin\ r}$$
From graph it is clear that $$\tan 30^{o}=\dfrac{\sin r}{\sin i}$$
$$\Rightarrow \dfrac{1}{\sqrt{3}}=\dfrac{\sin r}{\sin i}=\dfrac{1}{\mu}\Rightarrow \mu=\sqrt{3}$$
Also, speed of light in the medium $$v=\dfrac{c}{\mu}=nc\Rightarrow n=\dfrac{1}{\mu}=\dfrac{1}{\sqrt{3}}=(3)^{-1/2}$$
A concave mirror of focal length $$15\ cm$$ forms an image having twice the linear dimensions of the object. The position of the object when the image is virtual will be
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$$22.5\ cm$$
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$$7.5\ cm$$
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$$30\ cm$$
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$$45\ cm$$
Explanation
Given,
$$f=-15\ cm$$; focal length
$$ m=\pm 2$$; magnification(Positive because image is virtual)
We know that-
$$\Rightarrow m=-\dfrac{v}{u}$$
$$\Rightarrow v=-2u.$$
By using mirror formula-
$$\dfrac{1}{f}=\dfrac{1}{v}+\dfrac{1}{u}$$
$$\Rightarrow \dfrac{1}{-15}=\dfrac{1}{(-2u)}+\dfrac{1}{u}$$
$$\Rightarrow u=-7.5\ cm$$
The absolute refractive indices of water glass and diamond are 1.77, 1.50, and 2.72 respectively, which medium is most optically denser?
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Water
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Glass
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Diamond
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None
Explanation
Diamond
The optically denser medium has a larger refractive index as compared to the optically rarer medium. In this medium speed of light is minimum.
A light ray containing both blue and red wavelengths is incident at an angle on a slab of glass. Which of the sketches in this fig represents the most likely outcome?
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$$ A $$
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$$ B $$
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$$ C $$
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$$ D $$
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None of them
Explanation
Answer (c). For any medium, other than vacuum, the index of refraction for red light is slightly lower (closer to 1 ) than that for blue light. This means that when light goes from vacuum (or air) into glass, the red light deviates from its original direction less than does the blue light. Also, as the light reemerges from the glass into vacuum (or air), the red light again deviates less than the blue light. If the two surfaces of the glass are parallel to each other, the red and blue rays will emerge traveling parallel to each other, but displaced laterally from one another. The sketch that best illustrates this process is $$ C $$.
Where can we see an image formed the lens of an object?
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On the object itself
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On the lens itself
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On the same side of the object
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On the opposite side of the object
Explanation
The image formed by a lens is on the
side opposite to that of the object, thus option $$D$$ is correct.
Choose whether true or false:-
The lenses which feel thicker in the middle than at the edges are converging lenses.
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True
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False
Explanation
Those lenses which feel thicker in the middle than at the edges are convex lenses which are also called converging lens, thus the statement is true.
Which of the following are the properties of lenses?
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Opaque
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Transparent
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Allows light to pass through
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Shiny
Explanation
A piece of transparent material (such as glass) that has two opposite regular surfaces either both curved or one curved and the other plane and that is used either singly or combined in an optical instrument for forming an image by focusing rays of light.
Why is convex lens used as a magnifying glass?
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It forms erect image
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It forms real image
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It forms virtual image
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It forms magnified image
Explanation
Magnifying glasses are used to form a bigger image of a smaller object, generally object kept in front of mirror between pole and focus. In this given location of an object, convex lenses forms bigger and erect images. So convex lens is used as a magnifying glass.
What does the distance PF denote?
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Focus
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Focal Plane
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Centre of curvature
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Focal length
Explanation
The point on the principal axis from which all the light rays which are parallel to the principal axis appear to diverge after reflection from the convex mirror is known as the focus of the convex mirror.
The distance between the pole $$P$$ and focus $$F$$ is called the focal length.
Principal focus is represented by
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R
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f
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P
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F
Explanation
The point where light coming parallel to the principal axis meet or appear to meet after reflection from the spherical mirror is called principal focus of the mirror.
Principal focus is represented by
F.
In a convex mirror, the reflected moves parallel to the principal axis ray appear to come from a point on the principal axis.
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True
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False
Explanation
In a convex mirror, the reflected moves parallel to the principal axis ray appear to come from a point on the principal axis known as focus.
A number of rays parallel to the principal axis falling on a concave mirror intersect at a point, called the principal focus.
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True
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False
Explanation
Principle focus of the mirror is defined as a point where all paraxial rays intersect after reflection from the mirror.
Hence given
statement is True.
For concave mirror, the focus is in ________ of the mirror.
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behind
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front
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above
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below
Explanation
As seen in figure.
Rays of light coming parallel to the principal axis of a concave mirror will appear to converge on a point in front of the mirror somewhere between the mirror's pole and its center of curvature. That makes this a converging mirror and the point where the rays converge is called the focal point or focus .
Hence the focus of concave mirror is in front of mirror.
For convex mirror, the focus is _______ the mirror.
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behind
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front
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above
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below
Explanation
For convex mirror, the focus is
behind
the mirror.
$$F$$ is focus in the diagram
The plane through the focus perpendicular to the axis of a mirror or lens is
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Focal length
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Focus
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Focal Plane
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Pole
Explanation
The plane through the focus perpendicular to the axis of a mirror or lens is
called Focal Plane.
An object 4 cm in size, is placed at 20 cm in front of a concave mirror
of focal length 15 cm. Which of the following correctly represents the object distance (u) and focal length (f), as per New Cartesian sign
convention?
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$$u= 20 cm, f =-15cm$$
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$$u=-20 cm, f =-15cm$$
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$$u=20 cm, f =15cm$$
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$$u=-20 cm, f =15cm$$
Explanation
Since object distance is kept in front of the concave mirror, it is taken to be negative. The focal length of the concave mirror is taken as negative.
Hence
$$u=-20 cm, f =-15cm$$
All the distances measured to the left of
the origin are taken as _________.
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zero
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positive
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one
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negative
Explanation
New Cartesian Sign Convention.
Distance along left of pole of mirror is taken as negative and distance along right of pole is taken as positive..
Distances measured perpendicular to and above the principal axis
are taken as ________.
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negative
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positive
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zero
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none of the above
Explanation
Distances measured perpendicular to and above the principal axis
(along + y-axis) are taken as
positive
.
All the distances measured to the ________ of the origin
are taken as positive.
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left
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below
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right
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None of the above
Explanation
All the distances measured to the
right
of the origin (along
+ x-axis) are taken as positive.
In the case of a convex mirror, the image distance is taken to be positive because:
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Image is formed in front of the mirror
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centre of curvature and focus lies behind the convex mirror
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Image is always formed behind the mirror
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None of these
A spherical mirror has focal length -10cm. What type of mirror is it likely to be?
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convex
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concave
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plane
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none of the above
Explanation
The mirror will be a concave mirror as the focal length for a concave mirror is taken to be negative.
In case of spherical mirrors, object distance is taken to be_______
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positive
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negative
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unity
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zero
Explanation
According to the sign convention, the reflecting surface of the spherical mirrors is always towards the left hand side. The o
bject is always placed in front of the mirror's reflecting surface. Hence object distance measured towards the left of the mirror is taken as negative.
Distances measured perpendicular to and below the principal axis are taken as ________.
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negative
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positive
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zero
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one
Explanation
Distances measured perpendicular to and below the principal axis
(along y-axis) are taken as
negative.
All distances parallel to the principal axis are measured from the centre of curvature
of the mirror.
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True
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False
Explanation
The statement is False.
All distances parallel to the principal axis are measured from the
pole of the mirror.
In case of lenses. Image distance is negative for _______.
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real image
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virtual image
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mirror image
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none of the above
Explanation
As seen in figure virtual images are formed on the left side of the lenses.
And according to new cartesian sign convention distance measured to left of centre of lens is taken as negative.
In lenses, all the distances measured to the ________ of the origin are taken as positive.
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Left
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Right
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Below
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None of the above
Explanation
By the sign convention rule, all the distances measured to the right of the origin are taken as positive.
Sign conventions for lenses and mirrors are different.
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True
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False
Explanation
Sign conventions for lenses and mirrors are not different.
The principal axis of the mirror is taken as the $$x-axis$$ of the coordinate system. The conventions are as follows:
$$(i)$$ The object is always placed to the left of the mirror. This implies that the light from the object falls on the mirror from the left-hand side.
$$(ii)$$ All distances parallel to the principal axis are measured from the pole of the mirror.
$$(iii)$$ All the distances measured to the right of the origin (along $$+ x-axis$$) are taken as positive while those measured to the left of the origin (along $$– x-axis$$) are taken as negative.
$$(iv)$$ Distances measured perpendicular to and above the principal axis (along $$+ y-axis$$) are taken as positive.
$$(v)$$ Distances measured perpendicular to and below the principal axis (along $$–y-axis$$) are taken as negative.
Distances measured perpendicular to and below the optical center are taken as ________.
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Zero
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Negative
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Positive
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None of the above
Explanation
According to the sign convention for spherical lenses, distances measured perpendicular to and below the optical center are taken as negative.
Focal length of concave lens is ________.
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positive
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negative
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zero
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none of the above
Explanation
Focal length of concave lens is
negative.
Distances measured perpendicular to and above the optical center are taken as ________.
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Negative
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Positive
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Zero
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None of the above
Explanation
Distances measured perpendicular to and above the optical center are taken as positive.
Focal length of convex lens is _________
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Negative
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Zero
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Unity
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Positive
Explanation
Focal length of convex lens is positive.
The convex lens is a lens that converges rays of light that convey parallel to its principal axis (i.e. converges the incident rays towards the principal axis) which is relatively thick across the middle and thin at the lower and upper edges. The edges are curved outward rather than inward.
For a lens, the image distance is positive for _____.
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Real image
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Virtual image
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Plane image
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None of these
Explanation
For a lens, the image distance is positive for a real image.
All distances parallel to the principal axis are measured from the pole of the mirror. All the distances measured to the right of the origin (along $$+ x-axis$$) are taken as positive while those measured to the left of the origin (along $$– x-axis$$) are taken as negative.
In lenses, the distances measured to the left of the origin are taken as _________.
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Zero
0%
One
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Positive
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Negative
Explanation
According to the new cartesian sign convention.
In lenses, the distances measured to the left of the origin are taken as negative.
The distances measured to the right of the origin are taken as positive.
A lens has focal length of -30 cm . Identify the lens.
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Convex
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Cylindrical
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Concave
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None of the above
Explanation
The focal length of a concave lens is negative according to sign convention as the light rays diverge on the opposite side from where the light ray is coming after passing through the lens.
If a man's face is 25 cm in front of concave having mirror producing erect image 1.5 times the size of face, focal length of the mirror would be
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75 cm
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25 cm
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15 cm
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60 cm
Explanation
Negative sign shows that mirror is concave.
A lens has focal length of $$+20 cm$$ . Identify the lens.
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Convex
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Concave
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Cylindrical
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None of the above
Explanation
The focal length of a convex lens is positive according to sign convention as the light rays converge on the opposite side from where the light ray is coming after passing through the lens.
In case of lenses, object distance is taken to be_______.
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Positive
0%
Zero
0%
Negative
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One
Explanation
In case of lenses, object distance is taken to be negative as the object is placed to the left of the lens
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