If the medium through which an electromagnetic wave propagates possesses uniform optical properties, then it passes through undisturbed. If, however, there are spatial variations in the medium's optical properties, the wave is scattered. That is, some of its energy is redirected away from it original course. These spatial variations can be on the molecular scale so that N2 and O2 (the principle components of air) will scatter electromagnetic energy. Note that in pure scattering (as opposed to absorption-fluorescence) the energy of the incident photon is never incorporated into the molecule.
The two types of scattering mechanisms considered herein are Rayleigh and Mie scattering. Both are applicable only to isotropic, homogeneous spheres, and in the case of Rayleigh scattering, the wavelength must be much larger than the size of the scatterer.
Although non-spherical particles are frequently encountered in the atmosphere (e.g.: cirrus clouds), the scattering theory governing them is beyond the scope of this study. Scattering by ellipsoids and cylinders is discussed by Van de Hulst (1969) and ellipsoids and spheroids by Holt et al. (1978). A method to handle scattering by particles of arbitrary shape was developed by Purcell and Pennypacker (1973). Finally, Bohren and Singham (1991) adopt a statistical approach for an ensemble of irregular particles. A comparison of these three methods on four different homogeneous particles was carried out by Hovenier et al. (1996). A collection of work on non-spherical non-homogeneous scatterers can be found in the Journal of Quantitative Spectroscopy and Radiative Transfer, 55(6), 1996.