NCERT Exemplar Class 10 Science Chapter 10

Q1. 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

Answer: (a) Concave mirror as well as convex lens
Explanation: When the source is placed at the focus of a concave mirror or convex lens, the reflected or refracted rays become parallel to the principal axis.

Q2. A 10 mm long awl pin is placed vertically in front of a concave mirror. A 5 mm long image is formed at 30 cm in front of the mirror. Find the focal length of the mirror.

(a) −30 cm
(b) −20 cm
(c) −40 cm
(d) −60 cm

Answer: (b) −20 cm
Explanation: Magnification = h₂/h₁ = 5/10 = 1/2
So, v/u = 1/2 ⇒ v = −30 cm ⇒ u = −60 cm
Using mirror formula: 1/f = 1/v + 1/u = (−1/30) + (−1/60) = −1/20
Therefore, f = −20 cm.

Q3. Under which of the following conditions can a concave mirror form an image larger than the actual object?

(a) When the object is at radius of curvature
(b) When object is placed at a distance less than its focal length
(c) When object is placed between focus and centre of curvature
(d) When object is placed beyond radius of curvature

Answer: (c) When object is placed between focus and centre of curvature
Explanation: In this position, the concave mirror forms a real, inverted and magnified image (larger than the object).

Q4. Figure 10.1 shows a ray of light as it travels from medium A to medium B. The refractive index of medium B relative to medium A is:

(a) √3 / √2
(b) √2 / √3
(c) 1 / √2
(d) √2

Answer: (a) √3 / √2
Explanation: From the figure, angle of incidence i = 60° and angle of refraction r = 45°.
Using Snell’s law: n = sin60° / sin45° = (√3/2) / (√2/2) = √3 / √2.

Q5. A light ray enters from medium A to medium B as shown in Fig. 10.2. The refractive index of medium B relative to medium A will be:

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

Answer: (a) greater than unity
Explanation: The ray bends towards the normal while entering medium B, which means medium B is optically denser. Hence refractive index > 1.

Q6. Beams of light are incident through holes A and B and emerge out through holes C and D respectively as shown in Fig. 10.3. 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

Answer: (a) A rectangular glass slab
Explanation: A rectangular glass slab causes lateral displacement but the emergent rays remain parallel to the incident rays, matching the figure.

Q7. A beam of light is incident through holes on side A and emerges out of the holes on the other face of the box as shown in Fig. 10.4. Which of the following could be inside the box?

(a) Concave lens
(b) Rectangular glass slab
(c) Prism
(d) Convex lens

Answer: (d) Convex lens
Explanation: Parallel rays converge to a point and then diverge after crossing. This behavior is characteristic of a convex lens.

Q8. 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

Answer: (a) A convex lens has 4 dioptre power having a focal length 0.25 m
Explanation: Power P = 1/f (in meters). For f = 0.25 m, P = 4 D. Convex lens has positive power.

Q9. Magnification produced by a rear view mirror fitted in vehicles is:

(a) less than one
(b) more than one
(c) equal to one
(d) can be more than or less than one

Answer: (a) less than one
Explanation: Rear view mirrors are convex mirrors which always form virtual, erect and diminished images. Hence magnification is less than 1.

Q10. 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
(d) more than 30 cm

Answer: (b) 30 cm in front of the mirror
Explanation: Parallel rays from Sun focus at focal point ⇒ f = 15 cm. For same size image, object must be placed at centre of curvature = 2f = 30 cm.

Q11. 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

Answer: (b) a convex mirror
Explanation: A convex mirror provides a wider field of view and forms diminished images, allowing the full image of a distant tall building to be seen.

Q12. 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

Answer: (b) very near to the focus of the reflector
Explanation: When the source is placed at the focus of a concave mirror, the reflected rays become parallel, producing a strong beam of light.

Q13. 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

Answer: (d) all mirrors irrespective of their shape
Explanation: The laws of reflection are universal and apply to all types of reflecting surfaces.

Q14. The path of a ray of light coming from air passing through a rectangular glass slab traced by four students are shown in Fig. 10.5. Which one of them is correct?

(a) A
(b) B
(c) C
(d) D

Answer: (b) B
Explanation: In a rectangular glass slab, the emergent ray is parallel to the incident ray with a lateral shift. Only diagram B shows correct behavior.

Q15. 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

Answer: (d) Glycerine
Explanation: The medium with highest refractive index bends light the most. Glycerine has higher refractive index than water, kerosene and mustard oil.

Q16. 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

Answer: (d) Fig. D
Explanation: A ray parallel to the principal axis reflects through the focus, and a ray passing through focus reflects parallel to the principal axis. Fig. D correctly follows mirror rules.

Q17. Which of the following ray diagrams is correct for the ray of light incident on a lens as shown in Fig. 10.7?

(a) Fig. A
(b) Fig. B
(c) Fig. C
(d) Fig. D

Answer: (a) Fig. A
Explanation: A ray passing through the optical centre of a lens emerges undeviated. Fig. A correctly shows this behavior.

Q18. 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

Answer: (c) Concave, plane and convex
Explanation: Concave mirror gives enlarged image (head), plane mirror gives same size (middle), convex mirror gives diminished image (legs).

Q19. 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

Answer: (d) Concave mirror, convex mirror, concave lens and convex lens
Explanation: All these devices form image of an object at infinity at their focus, which is highly diminished and nearly point sized.