Birefringence

From New World Encyclopedia
The letters on a paper covered by a calcite crystal show up as doubled. This is an example of the phenomenon of birefringence (double refraction).

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.

Birefringent materials are used in many optical devices that manipulate the polarization of light, such as wave plates, polarizing prisms, and color filters.

Calculation of birefringence

If the material has a single axis of anisotropy, (i.e. 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.

Examples of birefringent materials

Many plastics are birefringent, because their molecules are 'frozen' in a stretched conformation when the plastic is molded or extruded. For example, cellophane is a cheap birefringent material.

There are many birefringent crystals: birefringence was first described in calcite crystals by the Danish scientist Rasmus Bartholin in 1669.

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.

Cotton (Gossypium hirsutum) fiber is birefringent because of high levels of cellulosic material in the fiber's secondary cell wall.

Slight imperfections in optical fiber can cause birefringence, which can cause distortion in fiber-optic communication; see polarization mode dispersion.

Silicon carbide, also known as Moissanite, is strongly birefringent.

Birefringence can also arise in magnetic (not dielectric) materials, but substantial variations in magnetic permeability of materials are rare at optical frequencies.

The refractive indices of several (uniaxial) birefringent materials are listed below (at wavelength ~ 590 nm), from [1].

Material no ne Δn
beryl Be3Al2(SiO3)6 1.602 1.557 -0.045
calcite CaCO3 1.658 1.486 -0.172
calomel Hg2Cl2 1.973 2.656 +0.683
ice H2O 1.309 1.313 +0.014
lithium niobate LiNbO3 2.272 2.187 -0.085
magnesium fluoride MgF2 1.380 1.385 +0.006
quartz SiO2 1.544 1.553 +0.009
ruby Al2O3 1.770 1.762 -0.008
rutile TiO2 2.616 2.903 +0.287
peridot (Mg, Fe)2SiO4 1.690 1.654 -0.036
sapphire Al2O3 1.768 1.760 -0.008
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

Creating birefringence

While birefringence is often found naturally (especially in crystals), there are several ways to create it in optically isotropic materials.

  • Birefringence results when isotropic materials are deformed such that the isotropy is lost in one direction (ie, stretched or bent). Example
  • Applying an electric field can induce molecules to line up or behave asymmetrically, introducing anisotropy and resulting in birefringence. (see Pockels effect)
  • Applying a magnetic field can cause a material to be circularly birefringent, with different indices of refraction for oppositely-handed circular polarizations (see Faraday effect).

Biaxial birefringence

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 labelled nα, nβ and nγ.

The refractive indices of some trirefringent materials are listed below (at wavelength ~ 590 nm), from [2].

Material nα nβ nγ
borax 1.447 1.469 1.472
epsom salt MgSO4·7(H2O) 1.433 1.455 1.461
mica, biotite 1.595 1.640 1.640
mica, muscovite 1.563 1.596 1.601
olivine (Mg, Fe)2SiO4 1.640 1.660 1.680
perovskite CaTiO3 2.300 2.340 2.380
topaz 1.618 1.620 1.627
ulexite 1.490 1.510 1.520

Measuring birefringence

Birefringence and related optical effects (such as optical rotation and linear or circular dichroism) can be measured by measuring the 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.

Elastic birefringence

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.

Applications of birefringence

Birefringence is widely used in optical devices, such as liquid crystal displays, light modulators, color filters, wave plates, optical axis gratings, etc. It also plays 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.

See also

References
ISBN links support NWE through referral fees

  • Born, Max, and Emil Wolf. 1999. Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light. 7th ed. Cambridge, UK: Cambridge University Press. ISBN 0521642221.
  • Halliday, David, Robert Resnick, and Kenneth S. Krane. 2001. Physics. Vol. 2. 5th ed. New York: John Wiley. ISBN 0471401943.
  • Sharma, Kailash K. 2006. Optics: Principles and Applications. Burlington, MA: Academic Press. ISBN 0123706114.

External links

  • Birefrigent Materials. Department of Physics and Astronomy, Georgia State University. Retrieved April 19, 2007.
  • Birefringence. Eric Weisstein's World of Physics. Retrieved April 19, 2007.

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