Question

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

Answer 1

Correct Answer - b.
`P=(1)/(f_(1))+(1)/(f_(2))-(d)/(f_(1)f_(2))`
`0=(1)/(20)-(1)/(5)-(d)/(20(-5))`
`(d)/(100)=(1)/(5)-(1)/(20)=(4-1)/(20)=(3)/(20)`
or `d=15cm`