Question

What condition must be satisfied by two thin lenses kept in contact so that they form an achromatic doublet, i.e., a combination free from chromatic aberration?

Answer 1

The condition for two thin lenses kept in contact to form an achromatic doublet is as follows:

$\left(1\right)$ The two thin lenses must be of different material.
$\left(2\right)$ One lens must be convex and the other must be concave.