Question

A convex lens of focal length $f_1$ is kept in contact with a concave lens of focal length $f_2$. Find the focal length of the combination.

Answer 1

For convex lens, focal length, $f=f_1$ and for concave lens, the focal length, $f=-f_2$
The equivalent focal length of a combination of convex lens and concave lens is given as :
$\frac{1}{F}=\frac{1}{f_1}+\frac{1}{-f_2}$
$F=\frac{f_1{f}_2}{f_1-f_2}$