Q: A crown glass prism of refracting angle 8° is combined with a flint glass prism to obtain deviation without dispersion. If the refractive indices for red and violet rays for crown glass are 1.514 and 1.524 and for the flint glass are 1.645 and 1.665 respectively, find the angle of flint glass prism and net deviation.

Sol: The condition for deviation without dispersion is

(μ_{v} – μ_{R} )A – (μ_{v}‘ – μ_{R}‘)A’ = 0

(μ_{v} – μ_{R} )A = (μ_{v}‘ – μ_{R}‘)A’

$\large A’ = \frac{1.524-1.514}{1.665-1.645} \times 8^o $

A’ = 4°

For crown glass $\large \mu = \frac{1.645+1.665}{2} = 1.655 $

The net deviation (δ -δ’ )=(μ-1)A -(μ’-1)A’

= 0.159 × 8° – 0.655 × 4° = 1.53°