Birefringence - Definition and Applications

Optical birefringence is an inherent property of a material, which causes double refraction of a ray of light resulting in polarization of the light wave. Birefringence is seen in crystallographic materials that have different refractive indices with respect to different crystallographic directions. Birefringence occurs when light passes through a transparent, molecularly ordered material, which exhibits orientation-dependent differences in refractive indices.

Birefringence is caused by the anisotropic forces that bind the atoms of a crystal. Anisotropy refers to the non-uniform distribution of properties in different directions. Some mineral crystals have two different refractive indices and, hence, show birefringence. Examples of minerals that exhibit birefringence are quartz, tourmaline, and calcite.

Occurrence of Birefringence

Calcite crystals show some of the strongest birefringence effects. Light passing through a calcite crystal is split into two rays by refraction. Refraction of the light waves occurs in such a way that two refracted rays are mutually perpendicular and, also, perpendicular to the direction of wave propagation. The two rays are termed as slow and fast rays. The wave having greater refractive index is called the slow ray.

Types of Birefringence and its measurement

Birefringence is classified as intrinsic or stress-induced birefringence.

• Intrinsic Birefringence – This type of birefringence is caused by the anisotropy present in the crystals. The atomic arrangement of the crystal itself is the source of birefringence. Examples are calcite, tourmaline, etc
• Stress-Induced Birefringence – This type of birefringence is caused due to the stresses imposed on the material. Materials such as glass or plastics show strain birefringence

Birefringence can be quantified by measuring the changes in polarization of light waves. This method of measurement is called polarimetry. A special method, called dual polarization interferometry, is used to measure birefringence of lipid bilayers.

Applications of Birefringence

Birefringence finds use in the following applications:

• Polarizing prisms and retarder plates
• Liquid crystal displays
• Medical Diagnostics

References

• Optical Birefringence – Florida State University
• Crystallographic Origins of Birefringence – Georgia State University