ABSTRACT
Circumstances arise in which one has need to study the passage through matter of photons with wavelength from as long as hundreds of kilometers or more to as short as the size of a nucleus. In some plasma or astrophysical applications, and when the gas is mono-atomic rather than molecular, the material density can be far below that of conventional solids. The fact that low-energy photons do not interact with solids, liquids, and polyatomic gases in the same way as they do with free, isolated atoms is no surprise and is well appreciated. The actual scattering cross section from each atom therefore differs from that when the atom is isolated. A collisionless plasma is opaque to electromagnetic radiation of frequency less than the plasma frequency. The chapter discusses photoelectric absorption, then coherent Rayleigh scattering. It presents a fairly thorough set of graphs for Compton scattering.
