Information AboutBirefringence |
| CATEGORIES ABOUT BIREFRINGENCE | |
| polarization | |
| optical mineralogy | |
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Birefringence, or '''double refraction''', is the decomposition of a Ray of Light into two rays (the '''ordinary ray''' and the '''extraordinary ray''') when it passes through certain types of material, such as Calcite Crystal s, depending on the Polarization of the light. This effect can occur only if the structure of the material is Anisotropic . If the material has a single axis of anisotropy, (i.e. it is ''uniaxial'',) birefringence can be formalised by assigning two different Refractive Indices to the material for different polarizations. The birefringence magnitude is then defined by: : (1) where ''n''o and ''n''e are the refractive indices for polarisations perpendicular and parallel to the axis of anisotropy respectively. (These rays are labelled 'ordinary' and 'extraordinary' respectively.) Birefringence can also arise in magnetic, not dielectric, materials, but substantial variations in magnetic Permeability of materials are rare at optical frequencies. ELECTROMAGNETIC WAVES IN AN ANISOTROPIC MATERIAL More generally, birefringence can be defined by considering a dielectric Permittivity and a refractive index that are Tensors . Consider a Plane Wave propagating in an anisotropic medium, with a relative permittivity tensor ε, where the refractive index '''n''', is defined by '''n.n = ε'''. If the wave has an electric Vector of the form: : (2) where r is the position vector and ''t'' is time, then the Wavevector '''k''' and the angular frequency ω must satisfy Maxwell's Equations in the medium, leading to the equations: : (3a) : (3b) where ''c'' is the Speed Of Light in a vacuum. Substituting eqn. 2 in eqns. 3a-b leads to the conditions: | ||
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