I4

a) Consider a beam of radiation propagating in one direction through a uniform slab of mat- ter, which begins at s = 0. The matter consists of particles which absorb the light, but it does not contain any sources of light. The absorber particle number density is n, and the (frequency- dependent) cross section for absorption is o. Using the radiative transfer equation, show that the specific intensity of the beam at a distance s into the slab is Iy(s) = 1,(0) exp(-Tv).