Question

A point source of light is placed at 2f from a converging lens of focal length f. A flat mirror is placed on the other side of the lens at a distance d such that rays reflected from the mirror are parallel after passing through the lens again. If f = 30 cm, then d is equal to

(a) 15 cm

(b) 30 cm

(c) 45 cm

(d) 75 cm

Answer 1

Answer is : (c) 45 cm

Let S is the source placed at distance 2f (= 60cm).

Image of S is formed at S′ (OS′ = 60 cm). As light rays after reflection from mirror are parallel after passing through lens, this is possible when they cross through focus as shown in figure.

For plane mirror, MS" = MS′