Problem in prism

The principal section of glass prism is an isosceles $\triangle PQR$ with $PQ = PR$ . The face $PR$ is silvered. A ray is incident perpendicularly on face $PQ$ and after two reflections, it emerges from base $QR$ , normal to it. The angle of the prism is given by $\dfrac{\pi}{\alpha}$ rad. Find the value of $\alpha$