Question

A convex lens of focal length `25 cm` and a concave lens of focal length `20 cm` are mounted coaxially separated by a distance `d cm` . If the power of the combination is zero, `d` is equal to
A. `45`
B. `30`
C. `15`
D. `5`

Answer 1

Correct Answer - D
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))-(d)/(f_(1)f_(2))=0`
`= (1)/(25)+(1)/(-20)-(d)/(-500)=0=(20-25)/(500)=-(d)/(500)`
`d=5 cm`