Magnetic double refraction

Magnetic and electric double refraction in liquids. Its explanation by the suspension of unobservable little crystallites whose complete alignment is impeded by the heat motion. A. Cotton and H. Mouton, Bull. aoc. de phya., 1910, p. 189 P. Langevin, Le Radium, 7 (1910), 249 O. M. Corbino, Phya. Zeitachr., 11 (1910), 756. 

Cotton-Mouton effect. (magnetic double refraction). Double refraction produced in some pure liquids by a magnetic field transverse to the hght beam. 

The ability of anisotropic and anisometric particles to assume some co-orientation in external force fields is not only responsible for significant changes in scattering properties but also causes birefringence (double refraction), i.e., the average refractive indexes of two beams polarized in perpendicular planes happen to be different. The specific orientation of particles and birefringecne may be caused by the action of electric field (Kerr effect), magnetic field (Cotton-Mouton effect), or in the case of anisotropic particles by flow of medium (Maxwell effect) [25]. 

As molecular motion in a gas or liquid is free and random, the physieal properties of these fluids are the same no matter in what direetion they are measured. In other words, they are isotropic. True amorphous solids, beeause of the random arrangement of their constituent molecules, are also isotropic. Most crystals, however, are anisotropic, their mechanical, electrical, magnetic and optical properties can vary according to the direction in which they are measured. Crystals belonging to the cubic system are the exception to this rule their highly symmetrical internal arrangement renders them optically isotropic. Anisotropy is most readily detected by refractive index measurements, and the striking phenomenon of double refraction exhibited by a clear crystal of Iceland spar (calcite) is probably the best-known example. 

Buckingham, A. D., 8c Pople, J. A. (1956). A theory of magnetic double refraction. Proceedings of the Physical Society, Section B, 69,1133. 

A simple form of double refraction, which has long been known, is furnished by putting glm under strain, which (apart from a few kinds) becomes positively double refracting in the direction of the pressure In this case one can speak of strain double refraction The double refraction produced in bodies under the influence of electric or magnetic fields was also reckoned as accidental On the removal of the external cause the double refraction usually also dii ppears For so far as this is not the case, strains remain in the material, such strains are to be found in, for example, glass objects which have been cooled too rapidly ... 

A further description of this phenomenon is to be found in chap III, 3 b, p 109 and in vol II, chap IV, 7, p 115 chap V, 7, p 142 chap XIV, 3, p 693, 6 p 719 Magnetic and electric doul le refraction in liquid systems is also almost always an orientation double refraction See chap III, 3 c, p 113 If it concerns an orientation of molecules, then one has only to do with the proper double refraction of these molecules with particles of colloidal dimensions there is in addition a second component the shape double refraction ... 

With liquid crystab and sols the double refraction, which b produced under the influence of magnetic or electric fields, can also be a typical orientation double refraction when the molecules or tfie particles are oriented by the field. 

Finally a separate treatment is required for the double refraction of light in those colloids in which an orientation of anisotropic particles occurs through mechanical strain, streaming, an electric or a magnetic field By this orientation a colloid can obtain the optical properties of a uni- or biaxial anisotropic medium ... 

In a colloidal solution of anisotropic particles an electric or a magnetic field can also cause orientation and thereby double refraction. In this case the particles are oriented by the field with their longest axis parallel or perpendicular to the field direction while the BROwriian movement again disturbs this orientation. There is thus produced a Boltzmann distribution symmetrical about one axis whereby the colloid behaves in this case as a uniaxial doubly refractive body. 

It can be understood that it is a great simplification for the development of the theory that the axes of the intrinsic anisotropy as a rule coincide with those of the shape anisotropy. The experimental set-up is simpler than with streaming double refraction since the orientation is produced in this case by bringing the solution into an electric or a magnetic field. 

The Cotton-Mouton constant (magnetic double refraction) is 111 —na AX,... 

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