Two thin equiconvex lenses each of focal length 0.2 m are placed coaxially with their optic centres 0.5m apart. Then find the focal length of the combination.

Options

A. `-0.4` m
B. `0.4` m
C. `-0.1` m
D. `0.1` m

Answer

A

Explanation

`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))-(d)/(f_(1)f_(2))`
or `(1)/(f)=(1)/(0.2)+(1)/(0.2)-(0.5)/((0.2)(0.2))` or `(1)/(f)=5+5-0.5xx5xx5xx5`
or `(1)/(f)=10-12.5=-2.5` 0r `f=(-1)/(2.5)=-0.4` m