CBSE - Physics - Light Reflection and Refraction

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. (a) between the pole and the focus of the reflector
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 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 highlydiminished and point sized.