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.

(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 

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.

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.

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:

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.

n_dg = v₁/ v₂ = 1.6

Absolute refractive index of glass
= − =1.5
or,

v₁ = c / 1.5

Substituting the value of v₁ in equation (1) we get;

c / 1.5 v = 1.6

c / v₂  = 1.6 x 1.5
=2.4
So, absolute refractive index of diamond=2.4

 

 

 

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.

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).