A thin prism having refracting angle $10^{\circ}$ is made of glass of refractive 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 :

Question

$\begin{array}{1 1} (1) 4^{\circ} \\ (2) 6^{\circ} \\ (3)8^{\circ} \\ (4) 10^{\circ} \end{array}$