Editing Birefringence (section)

== Sources of optical birefringence ==
[[File:cmglee_London_Embassy_Gardens_pool_polariser.jpg|thumb|upright|View from under the [[Sky Pool, London]] with coloured fringes due to stress birefringence of partially polarised skylight through a circular polariser]]
While the best known source of birefringence is the entrance of light into an anisotropic crystal, it can result in otherwise [[optical isotropy|optically isotropic]] materials in a few ways:
* ''[[#Stress-induced birefringence|Stress birefringence]]'' results when a normally isotropic solid is stressed and deformed (i.e., stretched or bent) causing a loss of physical isotropy and consequently a loss of isotropy in the material's permittivity tensor;
* Form birefringence, whereby structure elements such as rods, having one refractive index, are suspended in a medium with a different refractive index. When the lattice spacing is much smaller than a wavelength, such a structure is described as a [[metamaterial]];
* By the [[Pockels effect|Pockels]] or [[Kerr effect]], whereby an applied electric field induces birefringence due to [[nonlinear optics]];
* By the self or forced alignment into thin films of [[amphiphile|amphiphilic]] molecules such as [[lipids]], some [[surfactants]] or [[liquid crystals]]{{Citation needed|date=November 2021}};
* ''[[Optical rotation|Circular birefringence]]'' takes place generally not in materials which are anisotropic but rather ones which are [[chiral]]. This can include liquids where there is an [[enantiomeric excess]] of a chiral molecule, that is, one that has [[Stereoisomerism|stereo isomers]];
* By the [[Faraday effect]], where a longitudinal magnetic field causes some materials to become ''[[Optical rotation|circularly birefringent]]'' (having slightly different indices of refraction for left- and right-handed [[circular polarization]]s), similar to [[optical activity]] while the field is applied.