## In This post you will get answer of Q.

A convex and a concave lens seperated by distance $d$ are then put in contact. The focal length of the combination

Solution:

## When two lenses are placed coaxially at a distance $d$ from each other, then equivalent focal length $(F)$ is given by

$frac{1}{F}=frac{1}{f_{1}}+frac{1}{f_{2}}-frac{d}{f_{1} f_{2}}$

When the lenses are put together.

$d=0$

Hence, the focal length of the combination will decrease.