A shaft of light passes through a prism with refracting angle and refractive index n. Let be

Question:

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