Question

A convex lens A of focal length 20 cm and a concave lens B of focal length 5 cm are kept along the same axis with a distance d between them. If a parallel beam of light falling on A leaves B as a parallel beam, then the distance d in cm will be

(a)  25

(b)  15

(c)  30

(d)  50

Answer 1

Correct option  (b)

Explanation:

In the absence of concave lens the parallel beam will be focussed at F₂ i.e. at a distance 20 cm. from lens A.

The focal length of concave lens is 5 cm. i.e. if this lens is placed at 5 cm from F₂ , then beam will become parallel. So d = 15 cm.