Two thin prisms are combined to form an achromatic combination. For prism I, `A=4^(@),mu_(R)=1.35, mu_(Y)=1.40,mu_(v)=1.42` For prism II, `mu_(R)^(')=1.7, mu_(Y)^(')=1.8 and mu_(V)^(')=1.9` . Find the prism angle of prism II and the net mean deviation.

Explanation

Condition for achromatic combination.
`theta = theta'`
`(mu_(V) - mu_(R))A = (mu'_(V) - mu'_(R)) A'`
`:. A' = ((1.42-1.35)4^(@))/(1.9-1.7) = 1.4^(@)`
`delta_(Net) = delta - delta' = (mu_(V) - 1)A - (mu'_(Y) - 1)A' = (1.40-1)4^(@) - (1.8 - 1)1.4^(@) = 0.48^(@)`