Appendixes
Thermal radiation in semitransparent media

Many radiation problems can be formulated by using a single dimension of space, especially when assuming a plane medium contained between two infinite parallel walls (slab): transmission in atmospheric layers, in a pane of glass or through porous plates, etc. In this configuration, the key assumption is that gradients are zero in all directions except the one perpendicular to the plane of the slab. The radiative transfer equation then reduces to (see remark 20, (65) ):