Electric double refraction

J. Kerr found phosphorus to have a positive double refraction in the electric field. K. Schultz-Sellack showed that an 8-mm. layer of a soln. of 10 parts of phosphorus in one of carbon disulphide absorbs 52 per cent, of the rays emitted from a lampblack surface at 100° and 57 per cent, of the heat rays from a coal-gas flame. 

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. 

Voigt or Cotton-Mouton effect Zeeman effect (transmission) Lo-Surdo-Stark effect Electric double refraction... 

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]. 

Another method, which also employs molecular polarization, is based on the so-called Kerr effect. Kerr (1875) observed that light passing through an isotropic medium in an electric field perpendicular to the direction of the light ray is subject to double refraction (birefringence), i.e. the refractive indices parallel and perpendicular to the field, and n, are different. Kerr found the relation... 

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. 

If a gas, a solution or a pure liquid is introduced between the plates of a charged condenser, the molecules strive, as already pointed out, to orientate themselves with the axis of their maximum polarizability or, if a permanent moment exists, with the axis of this moment, parallel to the direction of the field. Should the thermal agitation be such, however, that this orientation is effected only to a very small extent, the previously isotropic medium exhibits anisotropy which can be detected as double refraction on the passage of polarized light. This electric double refraction imposed by the presence of the external field is called the Kerr effect. The phenomenon is measured by the path difference AX, between the beam polarized in the direction of the field and that polarized perpendicular to the field. It is given by the equation... 

In the case of third-order nonlinear phenomena the materials can be centrosymmetric in its molecular structure. One example is the optical Kerr effect reported for the first time by J. Kerr in 1877 and 1878 exposing a material to an electric field the refraction index of an optical medium changes, proportional to the square of the applied field. A double refraction can be generated with the difference between Kerr and Pockels effect being that in the latter case the double refraction is linearly proportional to the electric field. 

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. 

---