A prism is made of glass of unknown refractive index. A parallel beam of light is incident on a face of the prism. By rotating the prism the minimum angle of deviation is measured to be $40 ^{\circ}$ What is the refractive index of the prism ?

Question

$\begin{array}{1 1} 1.632 \\ 1.532 \\ 1.432 \\ 1.232 \end{array}$