A thin prism having refracting angle `10^(@)` is made of glass of refracting index `1.42`. This prism is combined with another thin prism of glass of refractive index `1.7`. This combination produces dispersion without deviation. The refracting angle of second prism should be `:`

Options

A. `4^(@)`
B. `6^(@)`
C. `8^(@)`
D. `10^(@)`

Answer

B

Explanation

Here, `A = 10^(@), mu = 1.42, mu' = 1.7, A' = ?`
As combination produces no deviation, therefore,
`(mu - 1) + (mu' - 1) = 0`
or `A' = (-(mu - 1) A)/((mu' - 1)) = (-(1.42 - 1) 10^(@))/((1.7 - 1))`
`= (-(0.42) 10^(@))/(0.7) = - 6^(@)`
Negative sign indicates that the two prism are combined in opposition.