A thin prism of angle $ 6^{\circ}$ made up of glass of refractive index 1.5 is combined with another prism made up of glass of refractive index 1.75 to produce dispersion without deviation. The angle of second prism is :

Question

$\begin {array} {1 1} (1)\;7^{\circ} & \quad (2)9^{\circ} \\ (3)\;4^{\circ} & \quad (4)\;5^{\circ} \end {array}$

 

Comments

Please explain the reason behind the answer