Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is:

$\frac{f_1+f_2}{f_1 f_2}$

The correct answer is Option (4) → $\frac{f_1+f_2}{f_1 f_2}$

When two lenses focal length $f_1$ and $f_2$ are in contact and coaxial,

$\frac{1}{f_{eff}}=\frac{1}{f_1}+\frac{1}{f_2}$  ...(1)

and,

$P=\frac{1}{f}$   ...(2)

∴ By (1) and (2)

$P=\frac{1}{f_1}+\frac{1}{f_2}$

$=\frac{f_1+f_2}{f_1 f_2}$