Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is:
Answer & explanation
Correct answer: option 4
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)
$P=\frac{1}{f_1}+\frac{1}{f_2}$
$=\frac{f_1+f_2}{f_1 f_2}$