Angle of the prism

A parallel beam of light is incident from air at an angle $\alpha$ 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 $\alpha$ has a minimum value of $45^{\circ}$ . What is the angle $\theta$ of the prism in degrees?