Question

You are given prisms made of crown glass and flint glass with a wide variety of angles. Suggest a combination of prisms which will

(a) deviate a pencil of white light without much dispersion, 

(b) disperse (and displace) of pencil of white light without much deviation.

Answer 1

(a) For the condition of deviation without dispersion, the first prism should be of crown glass and the second prism of flint glass. The refracting angle of the flint glass prism should be smaller than that of crown glass prism because the flint glass prism disperses more. Due to such combination the dispersion due to the first is nullified by the second. 

(b) For dispersion without deviation, the flint glass prism of greater and greater angle is taken, so that deviations due to the two prisms are equal and opposite. In the combination, the angle of flint glass prism will still be smaller than of crown glass because flint glass has higher refractive index than that of crown glass.