44. What is meant by ‘Chromatic aberration’? A thin convex lens of focal length 30 cm and made of flint glass (dispersive power = 0.03) is kept in contact with a thin concave lens of focal length 20 cm and made of crown glass. Calculate the dispersive power of crown glass if the above said combination acts as an achromatic doublet. 

Chromatic Aberration: When white light is passed through a prism it splits up into seven colours, with violet deviated towards the base of the prism, with maximum deviation and red suffering minimum deviation.

A lens can be assumed to be made up of a number of prisms.

The refracting angle of the prism goes on decreasing at a uniform rate from its centre outwards to its periphery. When white light is passed through a lens it is dispersed into its constituent colours.

The deviation is different for different colours and hence they come to focus at different points on the principal axis. Due to this we get a coloured and blurred image. This defect is called chromatic aberration.

Given,

f₁ = 30 cm, w₁ = 0.03

f₂ = 20 cm, w₂ = ?

Focal length of the combination,



We know,