Two thin lenses of focal lengths `f_(1)` and `f_(2)` are in contact. The focal length of this combination is

Options

A. `f_(1) + f_(2)`
B. `(f_(1)f_(2))/(f_(1) + f_(2))`
C. `(1)/(2) (f_(1) + f_(2))`
D. `(f_(1) + f_(2))/(f_(1)f_(2))`

Answer

D

Explanation

Focal length of combination,
`(1)/(F)=(1)/(f_(1))+(1)/(f_(2))`
`F=(f_(1)f_(2))/(f_(1)+f_(2))`
Power of lens, `P=(1)/(F)=(f_(1)+f_(2))/(f_(1)f_(2))`