(a) Show that the spectral density of the radiant energy flux the arrives in the solid angle d is cu cos d/4, where

(a) Show that the spectral density of the radiant energy flux the arrives in the solid angle dΩ is cuω cosθ · dΩ/4π, where θ is the angle the normal to the unit area makes with the incident ray, and uω is the energy density per unit frequency range.
(b) Show that the sum of this quantity over all incident rays is ¼cuω.