When white light passes through a prism , it disperses into band of seven colours. But when two prism are combined in such a way that sum of angular dispersions of crown glass prism and flint glass prism is zero then such a combination is achromatic combination of prisms.

When white light goes through a prism, it splits into seven distinct bands. When two prisms are joined in such a way that the total of the angular dispersions of the crown and flint glass prisms is zero, the result is an achromatic prism combination.

Such type of combination of prisms is called achromatic combination [Fig.]. Let the refractive indices of the prisms are µ and µ’ and their refractive angles are A and A’ respectively. Vertices of two prisms are opposite and placed back to back.

A crown glass prism of refracting angle 6 o is combined with a flint glass prism to give an achromatic combination. Calculate the refracting angle of the flint glass prism. What deviation will the compound prism produce?

An achromatic thin prism or achromatic wedge provides deviation without dispersion. Opposing prisms made from two different glasses ( n d1 , 1 , P 1 and n d2 , 2 , P 2 ) are combined to force the dispersion between the F and C wavelengths to be zero.

An achromatic prism is made by combining two prisms P 1 (μ v = 1.532, μ r = 1.515) and P 2 (μ v = 1.666 and μ r = 1.650). μ represents the refractive index. If the angle of the prism P 1 is 10 o, the angle of the prism P 2 will be.

Such a performance can be achieved with essentially the same basic approach as for an achromatic lens doublet: one can combine two prisms made from different optical materials, which may be contacted to obtain a single prism.

An achromatic prism is made by combining two prisms P 1 (μ v = 1.532, μ r = 1.515) and P 2 (μ v = 1.666 and μ r = 1.650). μ represents the refractive index. If the angle of the prism P 1 is 10 o , the angle of the prism P 2 will be.