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The science of determining the polarization state of electromagnetic radiation (x-rays, light or radio waves). Radiation is said to be linearly polarized when the electric vector oscillates in only one plane. It is circularly polarized when the x-plane component of the electric vector oscillates 90° out of phase with the y-plane component.
To completely specify the polarization state, it is necessary to make six intensity measurements of the light passed by a quarter-wave retarder and a rotatable linear polarizer, such as a Polaroid or a Nicol prism. The retarder converts circular light into linear light.
Most starlight is unpolarized. However, atoms in the presence of a magnetic field align themselves at fixed, quantized angles to the field direction. Then the spectral lines they emit are circularly polarized when the magnetic field is parallel to the line of sight, and linearly polarized when the field is perpendicular. The light from sunspots is polarized because the magnetic fields impose some direction in the emitting gas. Other phenomena also remove isotropy and produce polarization. See Zeeman effect
Electrooptical devices are rapidly replacing rotating polarizers and fixed retarders. The magnetograph consists of a spectrograph to isolate the atomic spectral line for study; a Pockels cell, an electrooptic crystal whose retardance depends on an applied voltage; a polarizing prism to isolate the polarization state passed by the retarder; a pair of photocells to detect the transmitted light; and a scanning mechanism to sweep the solar image across the spectrograph entrance slit. Two photocells are needed to simultaneously measure left- and right-circular polarization. See Spectrograph
A magnetograph can be made sensitive to linear polarization, but the signal levels are about 100 times weaker for the inferred transverse fields than for longitudinal fields of comparable strength. To improve signal-to-noise levels, the spectrograph can be replaced with an optical filter having a narrow passband, and the photocells can be replaced with an array of photosensitive picture elements (pixels).
polarimetry(poh-lă-rim -ĕ-tree) The measurement of the degree and direction of polarization of a beam of light or other radiation, using an instrument known as a polarimeter.
methods of investigation based on the measurement of (1) the extent of polarization of light and (2) optical activity, that is, the magnitude of the rotation of the plane of polarization of light as the light passes through optically active substances. Since the magnitude of such rotation in solutions is dependent on the concentration, polarimetry is widely used to measure the concentration of optically active substances. Spec-tropolarimetry is the measurement of rotatory dispersion, the change in the angle of rotation with the change in the wavelength of light. Measurements are made with polarimeters and spec-tropolarimeters and make possible a study of the molecular structure of substances.
Optical activity is extremely sensitive both to changes in the structure of a substance and to molecular interaction. It therefore can provide valuable information about the nature of sub-stituents in molecules of both organic and complex inorganic compounds, as well as about the conformation and internal rotation of these molecules. The difficulties in a theoretical calculation of the optical activity of chemical compounds derive from the nonadditivity of the phenomenon. This prevents calculations on the basis of a simple scheme, as in the case, for example, of molecular refraction. Optical activity, a second-order effect obtained when the phase difference of a light wave at different points in a molecule is take into consideration, results from the interactions of the molecule’s own electrons. The effect of molecular interaction on optical activity is examined in the theory of polarizability, where the molecule is considered as a system consisting of anisotropically polarized atomic groups. A specific electrostatic interaction, or dipole moment, is induced by the wave in a given group as it passes through this system. The affected group in turn induces additional dipoles in the other groups.
The study of dispersion of optical activity, particularly in measurements in a region of anomalous dispersion in the fundamental absorption band, provides information about the structure of biopolymers.
REFERENCESVol’kenshtein, M. V. Molekuliarnaia optika. Moscow-Leningrad, 1951.
Vol’kenshtein, M. V. Molekula i zhizn’. Moscow, 1965.
Djerassi, C. Dispersiia opticheskogo vrashcheniia. Moscow, 1962. (Translated from English.)
Terent’ev, A. P. Organicheskii analiz. Moscow, 1966.