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

Options

A. `25`
B. `15`
C. `30`
D. `50`

Answer

B

Explanation

In absence of a concave lens, the parallel beam will be focussed at `F_(2)`, i.e., at a distance of `20` cm from the lens A. The focal length of the concave lens is `5` cm, i.e., if this lens is placed at `5` cm from `F_(2)`, then the beam will become parallel. So, d= `15` cm.