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.
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.