Exercise Multiple Choice Questions (MCQS) : Light-Reflection and Refraction

CBSE
Class 10 Science
NCERT Exemplar Solutions
Chapter 10
Light-Reflection and Refraction
Exercise
Multiple Choice Questions (MCQs)
1. Which of the following can make a parallel beam of light when light from a point
source is incident on it?
(a) Concave mirror as well as convex lens
(b) Convex mirror as well as concave lens
(c) Two plane mirrors placed at 90° to each other
(d) Concave mirror as well as concave lens
Ans. (a) Concave mirror as well as convex lens
Explanation: When a point source of light is at the focus of a concave mirror or a convex
lens, then emergent rays of light make a parallel beam of light. This is the reason; concave
mirror is used as reflector of headlights.
2. A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long

image of the awl pin is formed at 30 cm in front of the mirror. The focal length of this
mirror is
(a) – 30 cm
(b) – 20 cm
(c) – 40 cm
(d) – 60 cm
Ans. (b) – 20 cm
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Explanation: object size h=10 mm, image size h’=5 mm, image distance v= -30 cm, object
distance =?, f = ?
magnification=
or or
or u= -60
Now, it can be calculated as follows:
or or
(the negative sign confirms that the calculated focal length is
of a concave mirror
)
3. Under which of the following conditions a concave mirror can form an image larger
than the actual object?
(a) When the object is kept at a distance equal to its radius of curvature
(b) When object is kept at a distance less than its focal length
(c) When object is placed between the focus and centre of curvature
(d) When object is kept at a distance greater than its radius of curvature
Ans. (c) When object is placed between the focus and centre of curvature
Explanation: When object is placed between F and C, an enlarged image is formed beyond C.
4. Figure 10.1 shows a ray of light as it travels from medium A to medium B. Refractive
index of the medium B relative to medium A is
(a)
(b)
(c)
(d)
Ans. (a)
Explanation: Refractive index of medium B relative to medium A can be calculated as

follows:
by snell's law
nB/nA=sin60/sin45
5. A light ray enters from medium A to medium B as shown in Figure 10.2. The
refractive index of medium B relative to A will be
(a) greater than unity
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(b) less than unity
(c) equal to unity
(d) zero
Ans. (b) less than unity
Explanation: In this case, the ray of light bends away from normal when it enters from
medium A into medium B. This shows that medium B is optically rarer than medium A.
Hence, speed of light in medium B is more than that in medium A. So, ratio of speed of light
in medium A to speed of light in medium B will be less than one.
6. Beams of light are incident through the holes A and B and emerge out of box through
the holes C and D respectively as shown in the Figure10.3. Which of the following could
be inside the box?
(a) A rectangular glass slab
(b) A convex lens
(c) A concave lens
(d) A prism
Ans. (a) A rectangular glass slab
Explanation: In this case, incident rays fall perpendicularly on the point of incidence. A
rectangular glass slab would refract and then re-refract it..
7. A beam of light is incident through the holes on side A and emerges out of the holes
on the other face of the box as shown in the Figure 10.4. Which of the following could be
inside the box?
(a) Concave lens
(b) Rectangular glass slab
(c) Prism
(d) Convex lens
Ans. (a) Concave lens
Explanation: The incident rays are parallel and emergent rays are diverging. Hence, it is a
concave lens (diverging lens).
8. Which of the following statements is true?
(a) A convex lens has 4 dioptre power having a focal length 0.25 m
(b) A convex lens has –4 dioptre power having a focal length 0.25 m
(c) A concave lens has 4 dioptre power having a focal length 0.25 m
(d) A concave lens has –4 dioptre power having a focal length 0.25 m
Ans. (a) A convex lens has 4 dioptre power having a focal length 0.25m
Explanation: The positive sign with the focal length indicates that it is a convex lens.
9. Magnification produced by a rear view mirror fitted in vehicles
(a) is less than one
(b) is more than one
(c) is equal to one
(d) can be more than or less than one depending upon the position of the object in front of it
Ans. (a) is less than one
Explanation: Convex mirror is used in rear view mirrors. Convex mirror always makes
smaller images. Hence, magnification produced by rear view mirror is less than one.
10. Rays from Sun converge at a point 15 cm in front of a concave mirror. Where should
an object be placed so that size of its image is equal to the size of the object?
(a) 15 cm in front of the mirror
(b) 30 cm in front of the mirror
(c) between 15 cm and 30 cm in front of the mirror
(d) more than 30 cm in front of the mirror
Ans. (b) 30 cm in front of the mirror
Explanation: In this case, f=-15 cm and hence c= -30cm; because radius of curvature is
double the focal length. When and object is placed on C, its image is of the same size,
inverted and is formed on C.
11. A full length image of a distant tall building can definitely be seen by using
(a) a concave mirror
(b) a convex mirror
(c) a plane mirror
(d) both concave as well as plane mirror
Ans. (b) a convex mirror
Explanation: Field of view of a convex mirror is more than any other type of mirror. Hence,
a full length image of a distant tall building can definitely be seen by using this.
12. In torches, search lights and headlights of vehicles the bulb is placed
(a) between the pole and the focus of the reflector
(b) very near to the focus of the reflector
(c) between the focus and centre of curvature of the reflector
(d) at the centre of curvature of the reflector
Ans. (b) very near to the focus
Explanation: Reflectors of headlights and searchlights are in the shape of concave mirror.
When source of light is placed at the focus, the reflected light appears like a beam of light.
13. The laws of reflection hold good for
(a) plane mirror only
(b) concave mirror only
(c) convex mirror only
(d) all mirrors irrespective of their shape
Ans. (d) all mirrors irrespective of their shape
14. The path of a ray of light coming from air passing through a rectangular glass slab
traced by four students are shown as A, B, C and D in Figure 10.5. Which one of them is
correct?
(a) A
(b) B
(c) C
(d) D
Ans. (b) B
Explanation: When ray of light enters into glass(optically denser)from air, it bends towards
the normal. When ray of light from glass emerges into air, it bends away from normal. This
is correctly shown by figure B.
15. You are given water, mustard oil, glycerine and kerosene. In which of these media a
ray of light incident obliquely at same angle would bend the most?
(a) Kerosene
(b) Water
(c) Mustard oil
(d) Glycerine
Ans. (d) Glycerine
Explanation: Refractive index of water is 1.33, that of kerosene is 1.44, that of mustard oil is
1.46 and that of glycerine is 1.47.
16. Which of the following ray diagrams is correct for the ray of light incident on a
concave mirror as shown in Figure 10.6?
(a) Fig. A
(b) Fig. B
(c) Fig. C
(d) Fig. D
Ans. (d) Fig D
Explanation: In case of concave mirror, an incident ray parallel to the principal axis passes
through F after reflection.
17. Which of the following ray diagrams is correct for the ray of light incident on a lens
shown in Fig. 10.7?
(a) Fig. A.
(b) Fig. B.
(c) Fig. C.
(d) Fig. D.
Ans. (a) Fig. A
Explanation: In convex lens, the incident ray passing through F goes parallel to the principal
axis after refraction.
18. A child is standing in front of a magic mirror. She finds the image of her head
bigger, the middle portion of her body of the same size and that of the legs smaller. The
following is the order of combinations for the magic mirror from the top.
(a) Plane, convex and concave
(b) Convex, concave and plane
(c) Concave, plane and convex
(d) Convex, plane and concave
Ans. (c) Concave, Plane and convex
Explanation: When object is between F and P of concave mirror, an enlarged and erect
image is formed behind the mirror. So, the child is seeing her head in a concave mirror.
Plane mirror makes similar sized image as object. So, the middle portion of her body appears
to be of same size in mirror. A convex mirror makes smaller image and hence legs are
appearing smaller in mirror.
19. In which of the following, the image of an object placed at infinity will be highly
diminished and point sized?
(a) Concave mirror only
(b) Convex mirror only
(c) Convex lens only
(d) Concave mirror, convex mirror, concave lens and convex lens
Ans. (d) Concave mirror, convex mirror, concave lens and convex lens
Explanation: In case of all of the above, when an object is at infinity, image is highly
diminished and point sized.
Short Answer Questions
20. Identify the device used as a spherical mirror or lens in following cases, when the
image formed is virtual and erect in each case.
(a) Object is placed between device and its focus, image formed is enlarged and behind it.
Ans. Concave mirror
(b) Object is placed between the focus and device, image formed is enlarged and on the same
side as that of the object.
Ans. Concave lens
(c) Object is placed between infinity and device, image formed is diminished and between
focus and optical centre on the same side as that of the object.
Ans. Concave lens
(d) Object is placed between infinity and device, image formed is diminished and between
pole and focus, behind it.
Ans. Convex mirror
21. Why does a light ray incident on a rectangular glass slab immersed in any medium
emerges parallel to itself? Explain using a diagram.
Ans. When a ray of light enters from one rarer medium (say air) to another denser medium
(say glass), it bends towards the normal. When the same ray of light exits from second
medium into first medium, it bends away from the normal. In this case, the extent of bending
of ray at opposite parallels (air-glass interface and glass-air interface) is same. Due to this, the
emergent ray is parallel to incident ray.
22. A pencil when dipped in water in a glass tumbler appears to be bent at the interface
of air and water. Will the pencil appear to be bent to the same extent, if instead of
water we use liquids like, kerosene or turpentine? Support your answer with reason.
Ans. We know that pencil appears to be bent at the interface of air and water because of
refraction of light. The degree of refraction depends on refractive index of a given liquid.
Refractive indices of kerosene, water and other liquids would be different. Hence, degree of
bend would be different in case of different liquids.
23. How is the refractive index of a medium related to the speed of light? Obtain an
expression for refractive index of a medium with respect to another in terms of speed
of light in these two media?
Ans. The refractive index can be seen as th factor by which the speed and the wavelength of
the radiation are reduced with respect to their vacuum values.
n=c/v (where n:refractive index,c=speed of light,v:velocity of light in that medium)
Refractive index of one medium in relation to a second medium is given by ratio of speed of
light in second medium to speed of light in first medium.
If v1 is the speed of light in first medium and v2 is the speed of light in second medium, then
refractive index of first medium in relation to second medium can be given as follows:
24. Refractive index of diamond with respect to glass is 1.6 and absolute refractive
index of glass is 1.5. Find out the absolute refractive index of diamond.
Ans. Absolute refractive index of a medium shows the ratio of speed of light in air to speed
of light in that medium. Let us assume c is the speed of light in air, v1 is speed of light in glass
and v2 is speed of light in diamond.
Rdg=1.6 ----(1)
Absolute refractive index of glass ,Rga=1.5 -----(2)
multiplying equation (1) and (2) , Rda X Rga
So, absolute refractive index of diamond=2.4
25. A convex lens of focal length 20 cm can produce a magnified virtual as well as real
image. Is this a correct statement? If yes, where shall the object be placed in each case
for obtaining these images?
Ans. This is correct statement.
When an object is placed between F and F2 of a convex lens, its enlarged, inverted and real
image is formed beyond 2F2, i.e. on the other side of lens. So, for this we need to place the
object between 20cm and 40cm of the lens.
When an object is placed between F and O of a convex lens, its enlarged, erect and virtual
image is formed beyond F2, i.e. on the same side of lens. So, for this we need to place the
object at a distance less than 20 cm from the lens.
26. Sudha finds out that the sharp image of the window pane of her science laboratory
is formed at a distance of 15 cm from the lens. She now tries to focus the building
visible to her outside the window instead of the window pane without disturbing the
lens. In which direction, will she move the screen to obtain a sharp image of the
building? What is the approximate focal length of this lens?
Ans. Let us assume that the window pane is between F2 and infinity from this lens and this is
a convex lens. We know that when the object is between infinity and F2, its inverted and real
image is formed between 2F and 2F2.
Now, the distant building is at infinity from the lens. Its image would be formed at 2F. So, the
screen needs to be moved towards the lens in order to get a sharp image. Its approximate
focal length is 10cm (less than image distance in earlier case).
27. How are power and focal length of a lens related? You are provided with two lenses
of focal length 20 cm and 40 cm respectively. Which lens will you use to obtain more
convergent light?
Ans. Power of a lens is reciprocal to its focal length. So, smaller focal length means more
power. Out of the given lenses, the lens with 20 cm as focal length has more power than the
lens with 40 cm as focal length. The lens with higher power should be used to obtain more
convergent light.
28. Under what condition in an arrangement of two plane mirrors, incident ray and
reflected ray will always be parallel to each other, whatever may be angle of incidence.
Show the same with the help of diagram.
Ans. When two plane mirrors are at right angles to each other, incident ray and reflected ray
will always be parallel to each other, whatever may be the angle of incidence. Let us use
following figure to understand this.
ED and DC are two plane mirrors placed at right angles to each other. An is incident ray on
CD and MN is normal at point of incidence. NQ is reflected ray from N and is incident on
point Q.
QB is reflected ray and PQ is normal to ED.
(angle i=angle r)
(same as above)
(MN is normal on CD) ...(1)
(mirrors are at right angle)
Hence, in ;
(acute angles of a right triangle are complementary) …(2)
From equations (1) and (2);
Hence,
(because and are complementary)
Now, and are supplementary (PQ is normal on DE)
So,
Or,
Since corresponding angles are equal
Hence, proved
This means that incident ray and reflected ray will always be parallel; irrespective of value
of angle of incidence.
29. Draw a ray diagram showing the path of rays of light when it enters with oblique
incidence (i) from air into water; (ii) from water into air.
Ans.
1. the light slows down when it passes from a rarer medium into a denser medium.Also, the
light ray bends more towards the normal.(left image)
2.from a denser medium to a rarer medium the light ray bends away from the normal.(right
image)
Long Answer Questions
30. Draw ray diagrams showing the image formation by a concave mirror when an
object is placed
(a) between pole and focus of the mirror
Ans.
(b) between focus and centre of curvature of the mirror
Ans.
(c) at centre of curvature of the mirror
Ans.
(d) a little beyond centre of curvature of the mirror
Ans.
(e) at infinity
Ans.
31. Draw ray diagrams showing the image formation by a convex lens when an object is
placed
(a) between optical centre and focus of the lens
Ans.
(b) between focus and twice the focal length of the lens
Ans.
(c) at twice the focal length of the lens
Ans.
(d) at infinity
Ans.
(e) at the focus of the lens
Ans.
32. Write laws of refraction. Explain the same with the help of ray diagram, when a ray
of light passes through a rectangular glass slab.
Ans. Laws of refraction are as follows:

(a) The incident ray, the refracted ray and normal at the point of incidence; all lie in the same
plane.
(b) The ratio of sine of incidence to sine of refraction is a constant, for the light of a given
colour and for a given pair of media. This law is also called Snell’s Law.
ABCD is a glass slab. EF is incident ray which is incident on point O on air-glass
interface.
NO is normal and which is angle of incidence.
N’O’ is normal extended towards the glass slab and which is angle
of refraction.
OO’ is refracted ray from surface AB. It behaves like incident ray on surface CD.
the ray EF bends when it enters the slab to become OO'.
MO’ and O’M’ are normal on surface CD.
GH is the emergent ray.
which is angle of incidence at surface CD.
which is angle of refraction at surface CD.
It is observed that EF, NO and OO’ lie in the same plane; which is in accordance to the
first law of refraction.
It is also observed that which means emergent ray is parallel to incident
ray. This happens because the degree of bend at opposite surfaces of glass slab is
same.
33. Draw ray diagrams showing the image formation by a concave lens when an object
is placed
(a) at the focus of the lens
Ans.
(b) between focus and twice the focal length of the lens
Ans.
(c) beyond twice the focal length of the lens
Ans.
34. Draw ray diagrams showing the image formation by a convex mirror when an object
is placed
(a) at infinity
Ans.
(b) at finite distance from the mirror
Ans.
35. The image of a candle flame formed by a lens is obtained on a screen placed on the
other side of the lens. If the image is three times the size of the flame and the distance
between lens and image is 80 cm, at what distance should the candle be placed from the
lens? What is the nature of the image at a distance of 80 cm and the lens?
Ans. Magnification is 3 times and image distance v=80 cm
Magnification=
Or,
Or,
Since, object distance is taken as negative; as per sign convention so,
or,
Nature of image: Since image is formed on the other side of lens hence it is a real image and
inverted image.
Nature of Lens: Enlarged image is formed by convex lens and not by concave lens, so it is a
convex lens.
36. Size of image of an object by a mirror having a focal length of 20cm is observed to be
reduced to 1/3rd of its size. At what distance the object has been placed from the
mirror? What is the nature of the image and the mirror?
Ans. Since image size is 1/3 of object size, so image distance is 1/3 of object distance because
h’/h=v/u
Using the mirror formula, we can calculate object distance and image distance
or,
or,
or, u= -40cm
Using this, we can find image distance:
or,
or,
or,
But above value of image distance does not match with our initial assumption. This means
that the mirror is not a concave mirror but a convex mirror. Let us calculate with the
assumption that it is a convex mirror.
or,
or,
or,
Nature of mirror: convex mirror
Nature of image: Smaller than object, erect and virtual
37. Define power of a lens. What is its unit? One student uses a lens of focal length 50
cm and another of –50 cm. What is the nature of the lens and its power used by each of
them?
Ans. The degree of divergence or convergence achieved by a given lens is called power of the
lens. The unit of power of lens is diopter and is expressed by D.
Focal length of lens used by first student is in positive hence it is a convex lens. The lens of
second student is a concave lens.
Power of lens (first student) =+2
Power of lens (second student) =-2
38. A student focussed the image of a candle flame on a white screen using a convex
lens. He noted down the position of the candle, screen and the lens as under
Position of candle = 12.0 cm
Position of convex lens= 50.0 cm
Position of the screen = 88.0 cm
(i) What is the focal length of the convex lens?
Ans.
Image distance v = 88 - 50 = 38 cm
Focal length can be calculated using the lens formula;
or,
or,
(ii) Where will the image be formed if he shifts the candle towards the lens at a position of
31.0 cm?
Ans. object distance u = 50 – 31 = 19 cm
In this case, object distance = focal length
This means that images is formed at infinity.
(iii) What will be the nature of the image formed if he further shifts the candle towards the
lens?
Ans. If he further shifts the candle towards the lens, the object comes between F and O. In
this case, image is virtual, enlarged and erect and is formed on the same side of lens.
(iv) Draw a ray diagram to show the formation of the image in case (iii) as said above.
Ans.

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Multiple Choice Questions (MCQs)

(a) Concave mirror as well as convex lens

(b) Convex mirror as well as concave lens

(c) Two plane mirrors placed at 90° to each other

(d) Concave mirror as well as concave lens

Ans. (a) Concave mirror as well as convex lens

2. A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long

Now, it can be calculated as follows:

(the negative sign confirms that the calculated focal length is

Explanation: Refractive index of medium B relative to medium A can be calculated as

(a) greater than unity

(c) equal to unity

Ans. (b) less than unity

(a) A rectangular glass slab

(b) A convex lens

(c) A concave lens

Ans. (a) A rectangular glass slab

(a) Concave lens

(b) Rectangular glass slab

(d) Convex lens

Ans. (a) Concave lens

8. Which of the following statements is true?

(a) is less than one

(b) is more than one

(c) is equal to one

Ans. (a) is less than one

(a) 15 cm in front of the mirror

(b) 30 cm in front of the mirror

(c) between 15 cm and 30 cm in front of the mirror

(d) more than 30 cm in front of the mirror

Ans. (b) 30 cm in front of the mirror

(a) a concave mirror

(b) a convex mirror

(c) a plane mirror

Ans. (b) a convex mirror

(a) between the pole and the focus of the reflector

(b) very near to the focus of the reflector

(c) between the focus and centre of curvature of the reflector

(d) at the centre of curvature of the reflector

Ans. (b) very near to the focus

13. The laws of reflection hold good for

(a) plane mirror only

(b) concave mirror only

(c) convex mirror only

(d) all mirrors irrespective of their shape

Ans. (d) all mirrors irrespective of their shape

(c) Mustard oil

Ans. (d) Fig D

Ans. (a) Fig. A

(a) Plane, convex and concave

(b) Convex, concave and plane

(c) Concave, plane and convex

(d) Convex, plane and concave

Ans. (c) Concave, Plane and convex

(a) Concave mirror only

(b) Convex mirror only

(c) Convex lens only

image formed is virtual and erect in each case.