Two thin lenses of focal length `f_(1)` and `f_(2)` are in contact and coaxial. The power of the 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

(d) Focal length of combinarion,
`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))`