Question

A convex lens is in contact with a concave lens. Focal length of both lenses are equal. What is the focal length of the combination ?

Answer 1

$\frac{1}{F}=\frac{1}{f_1}+\frac{1}{f_2}=\frac{1}{f}-\frac{1}{f}=0$
$\therefore f=\frac{1}{0}=\infty$
or $f=\infty \left(infinite\right)$