Consider a horizontal plane-parallel slab of gas of thickness L that is maintained at a constant temperature T. Assume that the gas has optical depth τλ,0, with τλ=0 at the top surface of the slab. Assume further that no radiation enters the gas from outside. Use the general solution of the transfer equation (9.54) to show that when looking at the slab from above, you see blackbody radiation if τλ,0≫1 and emission lines (where jλ is large) if τλ,0≪1. You may assume that the source function, Sλ, does not vary with position inside the gas. You may also assume thermodynamic equilibrium when τλ,0≫1.