Birefringence, or double refraction, is the splitting of a ray of light into two rays when it passes through certain types of material, such as calcite crystals. The two rays, called the ordinary ray and the extraordinary ray, travel at different speeds. Thus the material has two distinct indices of refraction, as measured from different directions. This effect can occur only if the structure of the material is anisotropic, so that the material's optical properties are not the same in all directions.
Birefringent materials are used in many optical devices, such as wave plates, liquid crystal displays, polarizing prisms, light modulators, and color filters.
Birefringence was first described in calcite crystals by the Danish scientist Rasmus Bartholin in 1669. Since then, many birefringent crystals have been discovered.
Silicon carbide, also known as Moissanite, is strongly birefringent.
Cotton (Gossypium hirsutum) fiber is birefringent because of high levels of cellulosic material in the fiber's secondary cell wall.
Slight imperfections in optical fibers can cause birefringence, which can lead to distortion in fiber-optic communication.
Birefringence can be observed in amyloid plaque deposits, such as are found in the brains of Alzheimer's victims. Modified proteins such as immunoglobulin light chains abnormally accumulate between cells, forming fibrils. Multiple folds of these fibers line up and take on a beta-pleated sheet conformation. Congo red dye intercalates between the folds and, when observed under polarized light, causes birefringence.
If the material has a single axis of anisotropy, (that is, it is uniaxial), birefringence can be formalized by assigning two different refractive indices to the material for different polarizations. The birefringence magnitude is then defined by:
where no and ne are the refractive indices for polarizations perpendicular (ordinary) and parallel (extraordinary) to the axis of anisotropy, respectively.
The refractive indices of several (uniaxial) birefringent materials are listed below (at a wavelength of about 590 nm).
|lithium niobate LiNbO3||2.272||2.187||-0.085|
|magnesium fluoride MgF2||1.380||1.385||+0.006|
|peridot (Mg, Fe)2SiO4||1.690||1.654||-0.036|
|sodium nitrate NaNO3||1.587||1.336||-0.251|
|tourmaline (complex silicate )||1.669||1.638||-0.031|
|zircon, high ZrSiO4||1.960||2.015||+0.055|
|zircon, low ZrSiO4||1.920||1.967||+0.047|
While birefringence is often found naturally (especially in crystals), there are several ways to create it in optically isotropic materials.
Birefringence and related optical effects (such as optical rotation and linear or circular dichroism) can be measured by measuring changes in the polarization of light passing through the material. These measurements are known as polarimetry.
A common feature of optical microscopes is a pair of crossed polarizing filters. Between the crossed polarizers, a birefringent sample will appear bright against a dark (isotropic) background.
Biaxial birefringence, also known as trirefringence, describes an anisotropic material that has more than one axis of anisotropy. For such a material, the refractive index tensor n, will in general have three distinct eigenvalues that can be labeled nα, nβ and nγ.
The refractive indices of some trirefringent materials are listed below (at wavelength ~ 590 nm).
|epsom salt MgSO4•7(H2O)||1.433||1.455||1.461|
|olivine (Mg, Fe)2SiO4||1.640||1.660||1.680|
Another form of birefringence is observed in anisotropic elastic materials. In these materials, shear waves split according to similar principles as the light waves discussed above. The study of birefringent shear waves in the earth is a part of seismology. Birefringence is also used in optical mineralogy to determine the chemical composition, and history of minerals and rocks.
Birefringence is widely used in optical devices, such as liquid crystal displays, light modulators, color filters, wave plates, and optical axis gratings. It plays an important role in second harmonic generation and many other nonlinear processes. It is also utilized in medical diagnostics. Needle biopsy of suspected gouty joints will be negatively birefringent if urate crystals are present.
All links retrieved June 10, 2016.
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