You are given prism made of crown glass and flint glass with a wide variety of angles. Suggest a combination of prism which will
(i) deviate a pencil of white light without much dispersion.
(ii) disperse and displace a pencil of white light without much deviation.

Explanation

(i) For no dispersion, total angular dispersion produced by two prisms should be zero.
i.e., `(mu_b - mu_r) A + (mu'_b - mu'_r) A' = 0`
As `(mu'_b - mu'_r)` for flint glass than that for crown glass, therefore, `A' lt A` i.e., flint glass prism of smaller angle has to be suitably combined with crown glass prism of larger angle.
(ii) For almost no deviation, `(mu_y - 1)A + (mu'_y - 1) A' = 0`
Taking crown glass prism of certain angle, we go on increasing angle of flint glass prism till this condition is met. In the final combination however, angle of flint glass prism will be smaller than the angle of crown glass as `mu'_y` for flint glass is more than `mu_y` for crown glass.