Problem 5.20

A shaft of light passes through a prism with refracting angle $\theta$ and refractive index $n .$ Let $\alpha$ be the diffraction angle of the shaft. Demonstrate that if the shaft of light passes through the prism symmetrically, (a) the angle $\alpha$ is the least; (b) the relationship between the angles $\alpha$ and $\theta$ is defined by Eq. $(5.1 \theta)$