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Polarized Light Microscopy

from: The Internet Pathology Laboratory

Some materials have the property of "birefringence" which is the ability to pass light in a particular plane. Such materials are called "anisotropic" because of this property. These are typically crystals or fibers. Normally, most materials are "isotropic" because any light that passes through them will be scattered in all directions. When viewed under polarized light, however, anisotropic materials will be brightly visible in one plane ("birefringent"), but will be dark in a plane turned 90 degrees.

The birefringence observed with polarized light can be further subdivided into "positive" and "negative" birefrigence. This is based upon the property of birefringence in which rays of light travelling through the anisotropic material in perpendicular planes (at right angles) will travel at different velocities through the material. Thus, a birefringent material actually has two refractive indices, a higher one for the "fast" rays of light and smaller refractive index for the "slow" rays travelling through the material. These rays of light can also be called "ordinary" when they are reflected by the material and "extraordinary" when the rays pass straight through the material.

A substance is positively birefringent if the "ordinary" reflected ray becomes the "fast" ray that travels faster in parallel with the crystalline structure of the material than the "extraordinary" ray that is "slow" when it traverses the material. Negative birefringence occurs when the "ordinary" ray becomes the "slow" ray when it is reflected and travels across the crystalline structure.





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