Physics Help Forum Hard Optics Question

 Oct 30th 2017, 11:47 AM #1 Junior Member   Join Date: Oct 2017 Posts: 1 Hard Optics Question A parallel beam of light is incident from air at an angle $a$ on the side $PQ$ of a right angled triangular prism of refractive index $n = \sqrt {2}$. Light undergoes total internal reflection in the prism at the face $PR$ when $a$ has a minimum value of $45°$. The angle $\theta$ of the prism is $...$ I'm so surprised for this question. May I get some help? Regards Here is my diagram