Birefringence

This effect was first described by Danish scientist Rasmus Bartholin in 1669, who observed it[2] in Iceland spar (calcite) crystals which have one of the strongest birefringences.

This causes an additional shift in that beam, even when launched at normal incidence, as is popularly observed using a crystal of calcite as photographed above.

In the second case the extraordinary ray propagates at a different phase velocity (corresponding to ne) but still has the power flow in the direction of the wave vector.

An anisotropic material is called "birefringent" because it will generally refract a single incoming ray in two directions, which we now understand correspond to the two different polarizations.

The index ellipsoid could still be described according to the refractive indices, nα, nβ and nγ, along three coordinate axes; in this case two are equal.

Thus the optic axis has the particular property that rays in that direction do not exhibit birefringence, with all polarizations in such a beam experiencing the same index of refraction.

For this reason, these crystals were designated as biaxial, with the two "axes" in this case referring to ray directions in which propagation does not experience birefringence.

In a birefringent material, a wave consists of two polarization components which generally are governed by different effective refractive indices.

[8] In other words, the polarization of the fast (or slow) wave is perpendicular to the optic axis when the birefringence of the crystal is positive (or negative, respectively).

Collagen, found in cartilage, tendon, bone, corneas, and several other areas in the body, is birefringent and commonly studied with polarized light microscopy.

[12] Inevitable manufacturing imperfections in optical fiber leads to birefringence, which is one cause of pulse broadening in fiber-optic communications.

Such imperfections can be geometrical (lack of circular symmetry), or due to unequal lateral stress applied to the optical fibre.

Birefringence is intentionally introduced (for instance, by making the cross-section elliptical) in order to produce polarization-maintaining optical fibers.

Birefringence can be induced (or corrected) in optical fibers through bending them which causes anisotropy in form and stress given the axis around which it is bent and radius of curvature.

Determination of the change in polarization state using such an apparatus is the basis of ellipsometry, by which the optical properties of specular surfaces can be gauged through reflection.

Waveplates are thin birefringent sheets widely used in certain optical equipment for modifying the polarization state of light passing through it.

By adjusting the angle of incidence, the effective refractive index of the extraordinary ray can be tuned in order to achieve phase matching, which is required for the efficient operation of these devices.

For instance, needle aspiration of fluid from a gouty joint will reveal negatively birefringent monosodium urate crystals.

The birefringence of tissue inside a living human thigh was measured using polarization-sensitive optical coherence tomography at 1310 nm and a single mode fiber in a needle.

Birefringence can be observed in amyloid plaques such as are found in the brains of Alzheimer's patients when stained with a dye such as Congo Red.

In ophthalmology, binocular retinal birefringence screening of the Henle fibers (photoreceptor axons that go radially outward from the fovea) provides a reliable detection of strabismus and possibly also of anisometropic amblyopia.

[23] Furthermore, scanning laser polarimetry uses the birefringence of the optic nerve fiber layer to indirectly quantify its thickness, which is of use in the assessment and monitoring of glaucoma.

[24] The same technology was recently applied in the living human retina to quantify the polarization properties of vessel walls near the optic nerve.

Dermoscopes use polarized light, allowing the user to view crystalline structures corresponding to dermal collagen in the skin.

The stress can be applied externally or is "frozen in" after a birefringent plastic ware is cooled after it is manufactured using injection molding.

3a can be expressed in terms of E through application of the permittivity tensor ε and noting that differentiation in time results in multiplication by −iω, eq.

When those two propagation constants are equal then the effective refractive index is independent of polarization, and there is consequently no birefringence encountered by a wave traveling in that particular direction.

Historically that accounts for the use of the term "biaxial" for such crystals, as the existence of exactly two such special directions (considered "axes") was discovered well before polarization and birefringence were understood physically.

And even when the optic axis is parallel to the surface, this will occur for waves launched at non-normal incidence (as depicted in the explanatory figure).

For a uniaxial crystal it will be found that there is not a spatial shift for the ordinary ray (hence its name) which will refract as if the material were non-birefringent with an index the same as the two axes which are not the optic axis.