Polarized light is created by passing light through a polarizing filter. This transmits light only in one direction. There are two polarizing filters in a polarizing microscope – above and below the sample (the polarizer and analyzer). The way in which materials interact with polarized light can provide information about their structure and composition.
About 90% of all solid substances have optical properties that vary with the orientation of the incident light (anisotropic materials). When these anisotropic materials are rotated, the observer can see brightness and/or color changes (pleochroism) under polarized light that depend on the orientation of the material in the optical path. These changes can be used to characterize and identify various materials. Isotropic materials, including stress-free glasses and cubic crystals, demonstrate the same optical properties in all directions.
Polarizing microscopy can be used with reflected and transmitted light. Reflected light is useful for studying opaque materials such as mineral oxides and sulfides, metal and silicone wafers that require stress-free objectives that have not been corrected for viewing through a coverslip.
Polarized light microscopy is perhaps best known for its geological applications – primarily for the study of minerals in thin sections of rocks but can also be used to study a variety of other materials including natural and industrial minerals (refined, mined or manufactured), composites such as cements, ceramics, mineral fibers and polymers, and crystalline or highly ordered biological molecules such as DNA, starch, wood and urea. The technique can be used both qualitatively and quantitatively in the materials sciences, geology, chemistry, biology, metallurgy, and medicine.
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