Light - Reflection and Refraction: Class 10 Important Questions with Answers

Solving mirror numericals — 4 steps

Case-based questions (4 marks each)

(i) Why is a concave mirror used? (1) (ii) Find the image position and magnification. (2) (iii) Will the image form on a screen? Why? (1)

(i) Find the focal length. (1) (ii) Where should the slide be placed? (2) (iii) Why is the slide put upside down? (1)

(i) Name the phenomenon. (1) (ii) Does light bend towards or away from the normal on leaving water? (1) (iii) Find the speed of light in water and explain why the pool looks shallower. (2)

(i) Net power? (1) (ii) Nature and focal length of the combination. (1) (iii) An object is 1 m from this combination; where is the image? Is it real? (2)

20 MCQs (1 mark each)

1. Image in a plane mirror is
   (a) real, inverted (b) virtual, erect, laterally inverted (c) real, erect (d) virtual, inverted
2. For a spherical mirror, R and f are related as
   (a) R = f (b) R = f/2 (c) R = 2f (d) R = 4f
3. Focal length of a concave mirror by sign convention is
   (a) positive (b) negative (c) zero (d) infinite
4. An object at C of a concave mirror forms an image
   (a) at F (b) at C, same size (c) behind (d) at infinity
5. Shaving mirrors are
   (a) plane (b) convex (c) concave (d) cylindrical
6. Image by a convex mirror is always
   (a) real (b) enlarged (c) virtual, erect, diminished (d) inverted
7. If m = −2, the image is
   (a) virtual, enlarged (b) real, inverted, enlarged (c) real, diminished (d) virtual, diminished
8. A ray through the centre of curvature of a concave mirror
   (a) goes through F (b) returns along the same path (c) becomes parallel (d) is absorbed
9. Light bends towards the normal when going from
   (a) glass to air (b) water to air (c) air to glass (d) glass to water
10. The emergent ray from a glass slab is
    (a) perpendicular (b) parallel to the incident ray (c) along the normal (d) reflected
11. Refractive index of a medium is
    (a) v/c (b) c/v (c) c × v (d) c + v
12. Light travels slowest in
    (a) water (b) glass (c) diamond (d) air
13. A ray through the optical centre of a lens
    (a) bends towards F (b) goes undeviated (c) becomes parallel (d) reflects
14. To get a same-size real image, the object is put at ___ of a convex lens
    (a) F (b) 2F (c) between F and O (d) infinity
15. Magnifying glass is a
    (a) concave lens (b) convex lens with object inside F (c) convex mirror (d) plane mirror
16. Power of a lens of focal length 50 cm (convex):
    (a) +0.5 D (b) +2 D (c) −2 D (d) +50 D
17. SI unit of power of a lens:
    (a) metre (b) watt (c) dioptre (d) joule
18. Lenses of +3 D and −1 D in contact give
    (a) +4 D (b) +2 D (c) −2 D (d) −3 D
19. Lens formula is
    (a) 1/v + 1/u = 1/f (b) 1/v − 1/u = 1/f (c) v − u = f (d) m = −v/u
20. A concave lens with f = −20 cm has power
    (a) +5 D (b) −5 D (c) −0.05 D (d) +0.2 D

Answer key

Why: 16 → 1/0.5 m = 2 D · 20 → 1/(−0.2 m) = −5 D · 12 → diamond has the largest n.

Assertion–Reason (10)

(a) Both A and R true, R explains A · (b) Both true, R does not explain A · (c) A true, R false · (d) A false, R true

1. A: Convex mirrors are used as rear-view mirrors. R: They form erect images and have a wide field of view. → (a)
2. A: A concave mirror can form a virtual image. R: The object is placed between P and F. → (a)
3. A: The focal length of a convex mirror is negative. R: Its focus is behind the mirror. → (d)
4. A: A pencil in water looks bent. R: Light changes speed when it goes from water to air. → (a)
5. A: Kerosene is optically denser than water. R: Kerosene has a lower mass density than water. → (b)
6. A: A ray falling normally on a glass slab does not bend. R: The angle of incidence is zero. → (a)
7. A: A concave lens always forms a diminished image. R: A concave lens is a converging lens. → (c)
8. A: Covering half a convex lens makes the image dimmer. R: Fewer rays pass through the lens. → (a)
9. A: A lens of shorter focal length has more power. R: P = 1/f. → (a)
10. A: The magnification of a real image is positive. R: Real images are inverted. → (d)

Give reasons (2 marks each)

1. Concave mirrors are used in car headlights.

With the bulb at the focus, reflected rays go out as a strong parallel beam that lights the road far ahead. 2

2. The bottom of a pool looks raised.

Light from the bottom bends away from the normal on entering air; the eye traces the rays back to a higher point. 2

3. A diamond sparkles more than glass.

Its refractive index (2.42) is very high, so light slows and bends a lot inside and is reflected many times before coming out. 2

4. A convex lens is called a converging lens.

It bends parallel rays inward so they meet at the principal focus. 2

5. A real image can be taken on a screen but a virtual one cannot.

For a real image the rays actually meet; for a virtual image they only appear to meet, so no light reaches a screen there. 2