A shaft of light passing through a prism with refracting angle θ and refractive index n experiences a deviation angle α. The discussion centers on demonstrating that α is minimized when the light passes symmetrically through the prism, meaning the internal path is parallel to the unused face of the prism. This symmetry implies that the angles of refraction at both faces of the prism are equal and opposite. To prove this, one must relate the angles using the properties of triangles and set the derivative of the deflection angle with respect to the angles equal to zero. Ultimately, the key conclusion is that symmetrical passage through the prism results in the least deviation angle α.
