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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).2
  • 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).

Measuring birefringence by polarimetry

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

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).3

Materialnαnβnγborax1.4471.4691.472epsom salt MgSO4•7(H2O)1.4331.4551.461mica, biotite1.5951.6401.640mica, muscovite1.5631.5961.601olivine (Mg, Fe)2SiO41.6401.6601.680perovskite CaTiO32.3002.3402.380topaz1.6181.6201.627ulexite1.4901.5101.520

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, 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.

See also

  • Crystal
  • Light
  • Optics
  • Refraction

Notes

  1. ↑ Refraction Retrieved October 1, 2007.
  2. ↑ Example Retrieved October 1, 2007.
  3. ↑ Refraction Retrieved October 1, 2007.

References

  • Born, Max, and Emil Wolf. Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light. 7th ed. Cambridge, UK: Cambridge University Press, 1999. ISBN 0521642221
  • Elert, Glenn. The Physics Hypertextbook: Refraction hypertextbook.com, 2007. Retrieved September 3, 2019.
  • Halliday, David, Robert Resnick, and Kenneth S. Krane. Physics. Vol. 2, 5th ed. New York: John Wiley, 2001. ISBN 0471401943
  • Sharma, Kailash K. Optics: Principles and Applications. Burlington, MA: Academic Press, 2006. ISBN 0123706114

External links

All links retrieved June 10, 2016.

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

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