When unpolarized light is scattered by a rough surface, or by a cloud of gas, of particles or of crystals, it becomes partially linearly polarized, and the plane of the polarization is usually found to be either normal to the plane containing the incident and observation rays, or parallel to this plane. If the scattered light has intensities l and 12 polarized in planes normal and parallel to the ray plane, then the degree of polarization is defined as P = (Ii —12 )/(li +12 )' usually expressed as a percentage, or in parts per thousand. P is found to change with the angle V between the incident and observation rays, usually known in astronomy as the phase angle. A plot of P against V is found to give a curve characteristic of the surface. Typically, curves of polarization for a rough or particulate solid surface is as shown in Fig. 1 , which refers to telescopic observations of the Moon for several wavelengths. Many such curves have been obtained by telescopic observation of bodies in the Solar System. Extensive laboratory studies have also been developed and are available for interpretation of the observations. The bulk of these results can be found in the review papers and major publications listed in the references.