DOUBLE REFRACTION — splitting of a bunch of unpolarized light in optically anisotropic medium on two bunches.
D.'s phenomenon of l. it was opened by E. Bartolin in 1669 and it is described by it on the example of a crystal of the Icelandic spar (CaCO 3 ).
D.'s studying l. in fabrics at fiziol., gistopatol. and other researches can have important diagnostic value.
If the narrow beam of light falls on a surface optically anisotropic (see. Anisotropy ) a crystal, on escaping it it is possible to observe two beams, one of which call ordinary, and another unusual (fig.). In uniaxial crystals the ordinary beam submits usual, and unusual to more difficult Snell refraction laws. Ordinary (AVSD) and unusual (AVS1D1) beams differ in various rate of propagation and thereof various indexes of refraction, and also the fact that they are linearly polarized in two mutually perpendicular planes (see. Polarization ). The corner formed by these beams is called D.'s corner of l.
D.'s phenomenon of l. it is widely used for obtaining information on existence of optical anisotropy (e.g., owing to formation of kristallopodobny educations), in investigated biol, material, it allows to judge conformational transitions of macromolecules, reorganizations of membranes, a complex formation, etc. So, e.g., at microscopy of muscle fibers of cross-striped muscles of a myofibril find the correct alternation of ghost and dark lines. At the same time more dark (dense) strips possess D. in l., what indicates anisotropism of structure. Actin as well as a myosin, D. possesses l., however only in that case when it is connected with calcium ions and forms fibrilla. This fact allowed to assume that at reduction of muscles there is a transition of a globular form of actin in fibrillar with the participation of calcium ions.
Of l. it is observed also in bones, sinews and fastion. Optical anisotropy of collagenic fibers, however in friable connecting fabric D. of l is well-marked. it is found hardly owing to the confused arrangement of bunches. Elastic fibers in an unextended state almost do not find D. of l.
In nervous tissue myelin fibers are birefringent. At development y death of nerve fibrils their optical properties change. Research of changes of D. of l. in single nerve fibrils at excitement gave the grounds to connect these changes with conformational reorganizations of membranes in various phases of development of action potential (see. Carrying out nervous impulse ).
D.'s phenomenon of h.p. success is used for studying of structure nucleinic to - t.
In patol, D.'s conditions of l. it is observed in body tissues very often. Optically anisotropic substances can be laid in fabric directly or appear as a result of chemical transformations of other (mostly fatty) substances. Examples patol, processes at which emergence in fabrics optically of anisotropic substances is observed are: adjournment of cholesteric connections in vessels, a cornea of an eye, sinews and kidneys in old age; development of the special tumors called by xanthomas which consist of cells like the macrophages containing neutral fat and a large number of holesterinester in plasma.
D.'s phenomenon of l. it is observed also at a research of cuts of fabrics of tumors.
Bibliography Wolkenstein M. W. Molecules and life, M., 1965; Land-sberg G. S. Optika, M., 1976; Levin S. V. Structural changes of cellular membranes, page 172, L., 1976
B. A. Gulyaev.