Rotation of the Plane of Polarization

ROTATION OF THE PLANE OF POLARIZATION, a change in the direction (plane) of vibration of polarized light rays (see Optical Polarization). This property is possessed by: 1. All transparent bodies if they are placed in a magnetic field (magnetic rotation of the plane of polarization). For homologous series of organic compounds, the specific magnetic rotation of the plane of polarization increases from one member of the series to the next by one and the same constant value characteristic of the given series. 2. A number of crystals, e.g., quartz. 3. All chemical compounds containing asymmetric atoms, i.e., atoms connected to no fewer than four different atoms or chemical groups. Of greatest importance are compounds containing asymmetric carbon atoms (see Asymmetric carbon). The ability to rotate the plane of polarization is called optical activity, and the corresponding substances are called optically active. To detect the rotation of the plane of polarization, it is simplest to use so-called "Nicol prisms" (see Polarization and Polarimetry), one of which is called the polarizer and the other the analyzer. If, at a certain mutual position of both prisms (with "crossed prisms"), one passes monochromatic (i.e., of only one specific wavelength) light rays through them, the analyzer will not pass any light at all—the field of view will be dark. If one places an optically active substance, for example, a quartz crystal, in the path of the polarized rays, the field of view becomes bright, and to extinguish this light, it is necessary to turn the analyzer by a certain angle to the right or left. Quartz, therefore, rotates the plane of polarization. Depending on the direction in which the rotation occurs, the substance is called dextrorotatory or levorotatory. Different colored rays require a different rotation of the analyzer for their extinction. Thus, with a quartz plate 1 mm thick, the angle of rotation turns out to be 15° for red, 21° for yellow, 27° for green, 33° for blue, and 51° for violet. The greatest rotation is observed for rays corresponding to short wavelengths. The angle of rotation of the planes of polarization is directly proportional to the thickness of the layer of the optically active substance through which the polarized ray passes, and, if one is dealing with a solution, it is directly proportional to the concentration of the latter. To compare the optical activity of different substances, their specific rotation of the plane of polarization is determined. This refers to the rotation that would be observed with a layer thickness of 10 cm (one decimeter) and a concentration of the given substance equal to 1 g per 1 cubic cm. Knowing the specific rotation of the plane of polarization for a given substance, one can determine its concentration in a solution from the rotation of the plane of polarization. The widely used method of determining sugar by means of so-called saccharimeters and, in general, the determination of optically active substances by means of polarimetry are based on this. The specific rotation of the plane of polarization is denoted by the symbol [α], and it is indicated to which wavelength of light and to which temperature it refers; determinations are usually conducted using the light of a sodium flame (which gives rays with a wavelength corresponding to the D line of the spectrum), and then the specific rotation of the plane of polarization is denoted by [α]_D^t (t = temperature).