# NCERT Exemplar Class 10 Science Chapter 10 Light- Reflection and Refraction Solutions

NCERT Exemplar Solutions for Class 10 Science Chapter 10 Light- Reflection and Refraction covers all the important questions and answers as well as advanced level questions. It helps in learning about the Light, Normal rays, Incident rays, Reflected rays, Angle of incidence, angle of reflection, point of incidence and Laws of reflection.

The NCERT Exemplar solutions for class 10 science is very important for board exams. NCERT Exemplar Solutions for Class 10 Science Chapter 10 Light- Reflection and Refraction is provided by our experts. They prepared the best solutions which help the students in understanding the solutions in an easy way. This chapters also covers the other topics like Laws of refraction, characteristics of an image formed by the plane mirror, mirror formula, spherical mirrors and its types, lens, power of lens real image and inverted image.

 Chapter Name Chapter 10 Light- Reflection and Refraction Book Title NCERT Exemplar for Class 10 Science Related Study NCERT Solutions for Class 10 Science Chapter 10 Light- Reflection and Refraction Revision Notes for Class 10 Science Chapter 10 Light- Reflection and Refraction MCQ for Class 10 Science Chapter 10 Light- Reflection and Refraction Important Questions for Class 10 Science Chapter 10 Light- Reflection and Refraction Topics Covered MCQ Short Answers Questions Long Answers Questions

### Multiple Choice Questions

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

Solution
(d) Concave mirror as well as concave lens

The emergent rays are parallel when point source of a light is focused to a convex or concave mirror.

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

Solution
Size of object = O = + 10.0 mm = + 1.0 cm (as, 1 cm = 10 mm)
Size of Image size = I = 5.0 mm = 0.5 cm
Image distance, v = − 30 cm (as image is real)
Let, object distance = u
Focal length, f =?

Magnification, m = I/O
Magnification is given by m = -v/u

Therefore,

⇒ u = -60 cm
Focal length is given by,

⇒ f = -20 cm

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

Solution
(c) When object is placed between the focus and centre of curvature

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) √3 / √2
(b) √2 / √3
(c) 1/ √2
(d)√2

Solution
(a) √3 / √2

Refractive Index of B with respect to A

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
(b) less than unity
(c) equal to unity
(d) zero

Solution
(b) less than unity
The ray of light is bending away from normal as it enters from medium A into medium B. This means that medium B is optically rarer than medium A. Hence, the speed of light in medium B is greater than the speed of light in medium A. Therefore, the 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

Solution
(a) A rectangular glass slab
A rectangular glass slab refracts and then re-refracts the rays when incident rays fall perpendicularly at the point of incidence on a rectangular glass slab.
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
Solution
(a) Concave lens
The incident rays are parallel and emergent rays are diverging.
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
Solution
(a) A convex lens has 4 dioptre power having a focal length 0.25 m
Convex lens has a positive focal length.
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
Solution
(a) is less than one
Rear view mirrors use convex mirrors. Convex mirror always form smaller image. Therefore, the magnification of rear view mirror is always less than 1.
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
Solution
(d) more than 30 cm in front of the mirror
Here,
focal length, f = 15cm,
Radius of curvature is twice the focal length.
Therefore, R =2 × 15 = 30cm
When an object is placed at R, the image formed is of the same size as that of the object. The image is formed at R and it is inverted.
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
Solution
(b) a convex mirror
Field of convex mirror is more than any type of mirror.
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
Solution
(b) very near to the focus of the reflector
Concave mirrors are used in headlight reflectors and search lights. When the source of light is placed at the focus, the reflected light appears like a beam.
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
Solution
(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

Solution
(b) B
When light passes from air to glass, it bends towards the normal and when light passes form glass to air it bends away from normal.
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
Solution
(d) Glycerine
The refractive indices of water, kerosene, mustard oil and glycerine are 1.33, 1.44, 1.46 and 1.47 respectively. Glycerine is optically the densest and hence a ray of light will bend more in glycerine.
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

Solution
(d) Fig. D
For a concave mirror, an incident ray parallel to 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

Solution
(a) Fig. A
In a convex lens, the incident ray passing through F becomes 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
Solution
(c) Concave, plane and convex
When the object is between F and P of concave mirror enlarged image is formed behind the mirror. Hence, child can see her head bigger in a concave mirror. She can see her body size of the same size because plane mirror gives image of original size. Convex mirror gives diminished images and babies legs appear smaller.
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
Solution
(d) Concave mirror, convex mirror, concave lens and convex lens

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.
(b) Object is placed between the focus and device, image formed is enlarged and on the same side as that of the object.
(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.
(d) Object is placed between infinity and device, image formed is diminished and between pole and focus, behind it.
21. Why does a light ray incident on a rectangular glass slab immersed in any medium emerges parallel to itself? Explain using a diagram.
Solution
When light ray enters denser medium from rarer medium it bends towards the normal. Again when this ray exists the second medium and enters the first, it ends away from the normal. In this case extent of bending of ray at opposite faces is the same, this is the reason why the emergent ray is parallel to the 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.
Solution
The bending of light takes place because of refraction. Refraction dependents on refractive indices of the medium. The refractive indices of kerosene or turpentine will not be same as that of water. Therefore, the degree of bend will be different in different mediums.
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?
Solution

The factor by which the speed and the wavelength of the radiation are reduced with respect to their vacuum values is known as the refractive index of the medium.
n = 𝑐/𝑣
(Where n: refractive index, c: speed of light, v: velocity of light in that medium)
Refractive index of one medium with respect to the second medium is given by ratio of speed of light in second medium to speed of light in first medium. 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.
Solution
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?
Solution
When an object is placed at F and F2 of a convex lens, an inverted, enlarged and real image is formed beyond 2F2 i.e. on the other side of the lens. Therefore, we need to place the object between 20 and 40 cm of the lens. When an object is placed between F and O of a convex lens, an enlarged, erect and virtual image is formed beyond F2 i.e. on the same side of lens. Therefore, 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?
Solution
Sudha will have to move the screen towards the lens in order to obtain a clear image of the building. The focal length of the lens will be approximately 15 cm. The ray of light coming from any distant object can be considered to be parallel. When parallel rays of light are incident on a convex lens, the rays, after refraction, converge at focus on the other side of the lens..
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?
Solution

P = 1/𝑓, P ∝1/𝑓. Therefore, the power of a lens is inversely proportional to its focal length. So, the lens having focal length of 20 cm will provide more convergence.

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.
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.
Solution
Speed of light decreases when it passes from rarer medium to denser medium and also light rays bend towards normal.
When ray of light passes from denser medium to rarer medium light rays bends away from the normal.

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
(b) between focus and centre of curvature of the mirror
(c) at centre of curvature of the mirror
(d) a little beyond centre of curvature of the mirror
(e) at infinity

Solution

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
(b) between focus and twice the focal length of the lens
(c) at twice the focal length of the lens
(d) at infinity
(e) at the focus of the lens
Solution

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.
Solution
Laws of refraction

1. Incident ray, refracted ray and normal at the point of incidence lie in the same plane.
2. Ratio of sine of incidence and sine of refraction is constant for the given color and pair of media.
ABCD is a glass slab. The incident ray is incident on point O on air-glass interface.
• NO is normal and ∠ION= i1; which is the angle of incidence.
• OM is the normal extended towards the glass slab and ∠O’OM= r1;which is angle of refraction.
• OO’ is refracted ray from surface AD. It behaves like incident ray for surface BC.
• N’O’ and O’M’ are normal on surface BC.
• O’E is the emergent ray.
• ∠OO’N’ = i2; which is angle of incidence at surface BC.
• ∠EO’M’ = r2; which is angle of refraction at surface BC.
• It is observed that the incident ray, the normal and the refracted ray lie in the same plane. This verifies the first law of refraction.
• It is also observed that IO is parallel to O’E; 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
(b) between focus and twice the focal length of the lens
(c) beyond twice the focal length of the lens

Solution
(a) The ray diagram when the object is placed at the focus of the concave lens :
(b) The ray diagram when the object is placed between focus and twice the length of focal length of the lens :
(c) Ray diagram when the object is beyond twice the focal length of the concave lens:

34. Draw ray diagrams showing the image formation by a convex mirror when an object is placed (a) at infinity
(b) at finite distance from the mirror

Solution
(a) At Infinity
(b) At infinite distance from the mirror

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?
Solution
As the image is obtained on the screen , it is real .
so,
Magnification , m = –3 ,
v = 80 cm
u = ?
As m = v/u
so,
–3 = 80/u,
u = –80/3 cm .
From

1/f = 1/20cm
So, f = 20 cm .
The lens is convex and image formed at 80 cm from the lens is real and inverted.

36. Size of image of an object by a mirror having a focal length of 20 cm 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?
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?
Solution
P =1/𝑓, where f is in metre.
The unit of power is Dioptre. Lens is convex in the first case and concave in the second case.
Power of lens (first student) =+2 dioptre
Power of lens (second student) = -2 dioptre
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?
(ii) Where will the image be formed if he shifts the candle towards the lens at a position of 31.0 cm?
(iii) What will be the nature of the image formed if he further shifts the candle towards the lens?
(iv) Draw a ray diagram to show the formation of the image in case (iii) as said above.
Solution
Position of the candle flame = 12.0cm
Position of the lens = 50.0 cm
Position of the screen = 88.0 cm
(i) u= 50-12= 38 cm.
Image distance v= 88-50= 38cm
Focal length = 1/𝑣 – 1/𝑢 = 1/𝑓
f= 19cm
(ii) Object distance u= 50 – 31= 19 cm
Here
Object distance = focal length
Hence the I mage is formed at infinity.
(iii) If he further shifts the candle towards the lens. The object comes between F and 0. In this case. Image is virtual, enlarged and erect and is formed on the same side of lens.