Polarization

Polarization of light, the ordered nature of light oscillations in a plane perpendicular to the direction of propagation of the light ray. The phenomenon of P. is thus closely connected with the transverse nature of light oscillations (see Light). In natural, non-polarized light, the oscillations occur completely chaotically, changing randomly both in magnitude and direction. In polarized light, the oscillations (in the plane perpendicular to the ray) occur either 1) in a straight line—plane P., or 2) in a circle—circular P., or 3) in an ellipse—elliptical P. The first case is called plane because in this case the oscillations, along the entire ray, lie in one plane. The plane of polarization is defined as the plane perpendicular to the plane in which the oscillations occur (see Light). P. of light can occur either during the emission process itself or as a result of the interaction of light with matter. All incandescent solids emit noticeably polarized light at certain angles (metals, glass, tourmaline). In resonance luminescence (of gases) and in fluorescence within them, polarized light is obtained. Among the phenomena of interaction of light with matter that produce P. are: scattering, reflection, refraction, the passage of light through crystals and similar media. The polarization of light when scattered by small particles is particularly noticeable in the direction of observation perpendicular to the scattered ray. An example of such P. is the polarization of the scattered light of the clear sky (here scattering occurs in the earth's atmospheric atmosphere). The simplest method of obtaining polarized light can be polarization upon reflection from a glass surface. In this case, plane polarized light is obtained. If the angle of incidence is approximately 57°, then complete polarization occurs. This angle is called Brewster's angle. A more complex method of obtaining plane polarized light is the method based on the sequential refraction of light through a stack of glass plates placed together. To obtain almost complete P., about a dozen plates are needed, but usually a smaller number of plates are used. The third method of obtaining plane polarized light is the method (or rather methods) based on the polarization of light during double refraction in crystals. The two rays resulting from double refraction are always plane polarized in two mutually perpendicular planes. If one of the rays is removed in any way, then upon exiting the crystal, plane polarized light is obtained. In a tourmaline crystal, one of the rays is simply absorbed by the crystal itself. In so-called polarization prisms, the removal of one of the rays is achieved artificially (see Nicol).-Elliptically polarized light can be obtained by passing plane polarized light through a crystal plate perpendicular to the optical axis of the crystal. Conversely, if elliptically polarized light is passed through such a plate, it will turn into plane polarized light (with the appropriate thickness of the plate).-Instruments used to obtain plane polarized light are called polarizers; they also serve as instruments for detecting P., and are called analyzers in this case. Instruments that convert light polarized in a circle or in an ellipse into plane polarized light are called compensators. Compensators are usually made of wedge-shaped plates to allow for smooth thickness changes.-In biological and medical sciences, the phenomenon of rotation of the plane of polarization (see) has found wide application. The magnitude of rotation is proportional to the thickness of the layer passed through and depends on the nature of the substance, its concentration, and the wavelength of the light passing through. If ρ is the density of the solution, α is the ratio of the weights of the dissolved substance and the solvent, l is the length of the liquid column, and α is the rotation in column l, then [α] = α/(ρ·l) is called the specific rotation, where l is in decimeters. Instruments called polarimeters (see Polarimetry) are used to measure the rotation of the plane of P.