Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is
Two thin lenses of focal lengths $f_1$ and $f_2$ are in contact and coaxial. The power of the combination is
$\sqrt{\frac{f_1}{f_2}}$
$\sqrt{\frac{f_2}{f_1}}$
$\frac{f_1+f_2}{f_1 f_2}$
$\frac{f_1+f_2}{f_1 f_2}$
Solution
If two thin lenses of focal lengths $f_1, f_2$ are placed in contact coaxially, then equivalent focal length of combination is
$\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}-\frac{0}{f_1 f_2}=\frac{1}{f_1}+\frac{1}{f_2}$
Power for the combination is
$P=\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{f_1+f_2}{f_1 f_2}$