(i) When a beam of white light is incident on a prism, the emergent beam is dispersed and is deviated from its original path. When two prisms of different materials and angles are combined, so as to produce dispersion without deviation, then such a combination is called direct vision prism. If the two prisms combined to produce deviation without dispersion, then such a combination is called a chromatic prism. For the deviation without dispersion, the angular dispersion produced by two prisms should be zero.
for flint glass is more than that for crown glass. So, A' < A i.e. the prism of flint glass is suitable with crown glass prism of larger angle.
(ii) For dispersion without deviation,
for flint glass is more than fly for crown
glass. So, A'< A, i.e. we use a flint glass prism of smaller angle with crown glass prism of larger angle.