The refractive index of the material of a prism
is $\sqrt 2 $ and the angle of the prism is 30°. One
of the two refracting surfaces of the prism is
made a mirror inwards, by silver coating. A
beam of monochromatic light entering the
prism from the other face will retrace its path
(after reflection from the silvered surface) if
its angle of incidence on the prism is