Although in gemology the term "birefringence" usually indicates the maximum difference between the ordinary and extra-ordinary rays (or the α and γ readings in biaxial stones), the term is also applied to any variation in refractive indices. {\displaystyle \delta \beta } cos → Voigt n E Δ . . The numerical difference between one RI value and the other RI value measured in any one case is called the "birefringence" for that test, and the difference between the highest possible RI and the lowest possible RI considering all possible directions is called the birefringence of the gemstone. in the same way with respect to the geometry in reflexion for the magnetization. 0 ∥ ⊥ i the electric field and a homogenously in-plane magnetized sample / e {\displaystyle {\vec {\nabla }}\times {\vec {H}}={\frac {1}{c}}{\frac {\partial {\vec {D}}}{\partial t}}} s y ( 0 {\displaystyle A/m} cos x {\displaystyle \delta \beta } c ( 489-493, Richard H. Cartier - Journal of Gemmology, 2002, Vol. is the induction defined from Maxwell's equations by c In this direction the light experiences zero refraction and maximum birefringence. there is no evidence that this approximation is not valid, as the observation of Voigt effect is rare because extremely small with respect to the Kerr effect. n r r → ) This gives another coercive field named. χ α cos i [1] Unlike many other magneto-optical effects such as the Kerr or Faraday effect which are linearly proportional to the magnetization (or to the applied magnetic field for a non magnetized material), the Voigt effect is proportional to the square of the magnetization (or square of the magnetic field) and can be seen experimentally at normal incidence. = Some gemstones have more than one refractive index (RI) because these stones belong to crystal systems (anisotropic) that have atomic structures that cause an incident ray of light to be resolved into two rays traveling at different velocities. i 2 It is defined as the bending of waves around the corners of an obstacle or through an aperture into the region of geometrical shadow of the obstacle/aperture. 0 0 A way to simplify the problem consists to use the electric field displacement vector → in the case of a paramagnetic material. In the approximation of no absorption, one obtains for the Voigt rotation in transmission geometry: As an illustration of the application of the Voigt effect, we give an example in the magnetic semiconductor (Ga,Mn)As where a large Voigt effect was observed. → i i y E H ) {\displaystyle B_{1}} This cycle was obtained by sending a linearly polarized light along the [110] direction with an incident angle of approximately 3° (more details can be found in [4]), and measuring the rotation due to magneto-optical effects of the reflected light beam. n Since t {\displaystyle \alpha } + 8, pp. {\displaystyle \chi } Diffraction refers to various phenomena that occur when a wave encounters an obstacle or opening. H {\displaystyle B_{1}=B_{2}} / and / 1 D E 0 M r i {\displaystyle L} 1 where is proportional to Here, one considers an incident polarisation propagating in the z direction: = 0 The resulting expression is then inserted in (8). ϵ ⊥ i are refracted) and move in different directions this phenomenon is called "double refraction. and the ellipticity angle Many times this doubling may be difficult to identify depending on the stone size, so don’t rely on that alone for identification. {\displaystyle M^{2}} and an in-plane polarized sample