# Double refraction

Optical mineral properties are so diverse that they represent reliable means for rapid diagnosis of minerals. First of all, the refractive index of minerals is determined, and then the double refraction, which is so brilliantly manifested in calcite, the optical sign of a crystal, etc.

In 1669 the Danish scientist Erasmus Bartolin published the work "Experiments with crystals of Icelandic calcareous spar, which reveal a surprising and strange refraction." He discovered the double refraction of light, studying calcite crystals. The images viewed through his crystals turned out to be bifurcated. Double refraction has its say in the famous dispute between Huygens and Newton about the nature of light: is it corpuscular or wave? In 1808, the Paris Academy of Sciences announced a competition for the best mathematical theory of birefringence of crystals. The competition won E. Malius (1775-1812), who discovered the polarization of light, as the solution to double refraction.

The essence of the phenomenon boils down to the fact that if a ray of light falls perpendicular to the surface of the crystal, then on this surface it splits into two rays. The first ray continues to propagate directly, and is called ordinary (o-ordinary), the second ray is diverted to the side, and is called extraordinary (e-extraordinary).

The violation of the law of refraction of light by an extraordinary beam is due to the fact that the speed of light propagation (and hence the refractive index) of an extraordinary ray depends on the direction. For an ordinary wave, the propagation velocity is the same in all directions. According to the classical theory of light, birefringence is due to the fact that the alternating electromagnetic field of light forces the electrons of matter to oscillate, and these oscillations influence the propagation of light in the medium, and in some substances, make electrons oscillate easier in certain definite directions. For example, crystals are called negative crystals, in which the propagation speed of an ordinary light beam is less than the propagation speed of an extraordinary beam. Positive crystals are crystals in which the propagation velocity of an ordinary light beam is greater than the propagation velocity of an extraordinary beam.

In addition to crystals, birefringence is also observed in isotropic media placed in an electric field (the Kerr effect), in a magnetic field (the Cotton-Mouton effect, the Faraday effect), under the action of mechanical stresses (photoelasticity). Under the influence of these factors, the initially isotropic medium changes its properties and becomes anisotropic.