# Problem in prism

Classical Mechanics Level pending

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 $$\alpha$$

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