Q 12-09-013NEETNEET 2021Top questionEasy
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 :
Answer: (C) $15$
Lens A brings the parallel beam towards its focus, $20$ cm beyond A. For B to send it out parallel, that point must be at B's (virtual) focus, $5$ cm beyond B.
$$d = 20 - 5 = 15\ \text{cm}$$
Solution by Sreeraj P, M.Sc Physics