(a) Show that for an incident radiation intensity of Iph, i (energy per unit area irradiated is Iph, i) in a medium with an absorption coefficient Oph, the divergence (in one dimension, along x) of radiation heat flux aph is equal to - Iph, ph expl-phx). In the positive direction, the radiation heat flux at x = 0 is from (7.32) as 4ph (x = 0) = 2 5 Iph.i cos 9 sin 9d9 Iph.i. Assume there is no emission and scattering. The divergence of aph appears in the macroscopic energy equation given (b) Comment on how this term becomes a heat source in the macroscopic energy equation when radiation is absorbed.