Chromatic aberration is an optical defect by virtue of which image of an object seen in white light through a lens appears coloured. It arises on account of inability of a lens to focus all the colours (in white light) at the same point. In Fig. a red rays are focussed at Fr and violet rays are focussed af F, all other colours being focussed in between. The difference in focal lengths of the lens for red and violet rays i.e. (fr - fv) is a measure of longitudinal chromatic aberration of the lens. It is positive in case of a convex lens and negative in case of a concave lens.
To remove chromatic aberration, we use an achromatic doublet. This is a combination of a convex lens of crown glass and concave lens of flint glass with their focal lengths adjusted suitably so that net chromatic aberration of the combination is zero.
The achromatic doublet will thus focus all the colours at the same point. Hence the image seen in white light shall not be coloured i.e. it will be free of chromatic aberration.
