Class 10 Science

# Light

## Exemplar Problems

### MCQ Part 2

Question 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?

1. 15 cm in front of the mirror
2. 30 cm in front of the mirror
3. between 15 cm and 30 cm in front of the mirror
4. more than 30 cm in front of the mirror

Answer: (b) 30 cm in front of the mirror

Explanation: When object is at C of concave mirror, image of same size is formed at C. The image is inverted and real.

Question 11: A full length image of a distant tall building can definitely be seen by using

1. a concave mirror
2. a convex mirror
3. a plane mirror
4. both concave as well as plane mirror

Explanation: An aperture of convex mirror is always large enough to make full length image of distant tall structures. Full length image may not be always obtained by using plane or concave mirrors.

Question 12: In torches, search lights and headlights of vehicles the bulb is placed

1. between the pole and the focus of the reflector
2. very near to the focus of the reflector
3. between the focus and centre of curvature of the reflector
4. at the centre of curvature of the reflector

Answer: (b) very near to the focus of the reflector

Explanation: When source of light is at focus of concave mirror, all the reflected rays are parallel to each other. This gives a straight beam of light.

Question 13: The laws of reflection hold good for

1. plane mirror only
2. concave mirror only
3. convex mirror only
4. all mirrors irrespective of their shape

Answer: (d) all mirrors irrespective of their shape

Question 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 following figure. Which one of them is correct?

1. A
2. B
3. C
4. D

Explanation: Incident ray and emergent ray are parallel in this option only.

Question 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?

1. Kerosene
2. Water
3. Mustard oil
4. Glycerine

Explanation: Ray of light bends at maximum angle in liquid having highest refractive index.

Refractive index of liquids given in the option:

Kerosene: 1.44
Water: 1.0003
Mustard oil: 1.464M
Glycerine: 1.473

Since, the refractive index of glycerine is maximum, thus light bends at maximum angle in glycerine.

Question 16: Which of the following ray diagrams is correct for the ray of light incident on a concave mirror as shown in following figure?

1. Fig. A.
2. Fig. B.
3. Fig. C.
4. Fig. D.

Explanation: When a ray of light parallel to principal axis is incident on a concave mirror, it passed through F after reflection.

Question 17: Which of the following ray diagrams is correct for the ray of light incident on a lens shown in following figure?

1. Fig. A.
2. Fig. B.
3. Fig. C.
4. Fig. D.

Explanation: When a ray of light passes through F of a convex lens, it goes parallel to the principal axis after refraction through the lens.

Question 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.

1. Plane, convex and concave
2. Convex, concave and plane
3. Concave, plane and convex
4. Convex, plane and concave

Answer: (c) Concave, plane, and convex

Explanation: Image size is same as object size in plane mirror. Image size is smaller than object size in convex mirror. Image size is larger than object size when object is between F and P of concave mirror.

Question 19: In which of the following, the image of an object placed at infinity will be highly diminished and point sized?

1. Concave mirror only
2. Convex mirror only
3. Convex lens only
4. Concave mirror, convex mirror, concave lens and convex lens

Answer: (d) Concave mirror, convex mirror, concave lens and convex lens

Explanation: When rays of light coming from infinity are incident on a concave mirror, convex mirror, concave lens or a convex lens, a highly diminished image is formed at the focus as shown in the figure.

The same can be verified by using mirror equation and lens equation.

Here, u=-∞

Mirror Formula: 1/v+1/u=1/f

Or, 1/v-1/(∞)=1/f

Or, 1/v=1/f

Or, v=f

Lens Formula: 1/vā1/u=1/f

Or, 1/v+1/(∞)=1/f

Or, 1/v=1/f

Or, v=f

Now, using the magnification formula:

m=v/u=f/ā=0

This proves that the image is point sized.

Note: All figures on this page are taken from NCERT Exemplar Problems Class 10 Science Book