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. 15
B. 50
C. 30
D. 25

Answer

A

Explanation

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