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 incident radiation of intensity Iλ,0 enters the bottom of the slab 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. If τλ,0≪1, show that you see absorption lines superimposed on the spectrum of the incident radiation if Iλ,0>Sλ and emission lines superimposed on the spectrum of the incident radiation if Iλ,0<Sλ. (These latter two cases correspond to the spectral lines formed in the Sun's photosphere and chromosphere, respectively; see Section 11.2.) 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.