Question

A convex lens of focal length 20cm and a concave lens of focal length f are mounted coaxially 5cm apart. Parallel beam of light incident on the convex lens emerges from the concave lens as a parallel beam. Then, f in cm is
A. 35
B. 25
C. 20
D. 15

Answer 1

Correct Answer - d.
Clearly, power of system is zero.
`:.0=(1)/(20)+(1)/(f)-(5)/(20f)`
or `-(1)/(20)=(15)/(20f) or f=-15cm`