A polarizing microscope, also known as a petrographic microscope, uses polarized light to study geological specimens and anisotropic materials. It has extra features such as polarizers, analyzers, and a Bertrand lens. Compensation and retard plates can also be used for better observation.
A polarizing microscope is a microscope used primarily in geological studies to study geological specimens. For this reason it is also known as a petrographic microscope. It is also used in other scientific fields such as medicine and biology.
Polarizing microscopes are built like a regular light microscope, but come with a few extra features. Unlike ordinary microscopes which use normal light, a polarizing microscope uses polarized light to study specimens. In polarized light, light waves vibrate in one direction; under normal light, light waves vibrate in random directions.
Polarized light cannot be seen by human eyes under normal circumstances. It can, however, be used in polarized light microscopy to highlight the characteristics of minerals and other materials. A polarizing microscope uses the birefringent optical properties of anisotropic materials to study them.
Anisotropic materials are solid substances that have different refractive indices; isotropic materials, which include gases and liquids, have only one refractive index. Birefringence or double refraction occurs when a light wave passing through an anisotropic material is split into two beams of different speeds.
Geological samples are cut or ground into thin sections for study. The sample to be studied is placed on a slide on a revolving sample holder stage. The sample is then illuminated by a light source under the sample stage.
The light passes through a polarizing filter called a polarizer and then passes through the birefringent sample. The polarizer is usually fixed in an east-west vibrational direction, but can be rotated as needed. There is another polarizing filter called an analyzer. It is usually located above the lenses and can be moved in and out of the optical path.
The objectives used in a polarizing microscope must be free from deformation. The eyepiece may have a cross-wire reticle or a photomicrograph reticle. The cross-wire reticle makes it easy to center in the sight. The photomicrographic graticule is useful for selecting an area for film capture.
Many polarizing microscopes have a Bertrand lens. It is located between the eyepiece and the objective. A Bertrand lens helps to study the back focal plane of the lens to discover the interference pattern.
Compensation and retard plates can also be used in polarized light microscopy to better observe optical path differences. These plates can be inserted into a slot in the eyepiece or into a tube between the body and the eyepiece tubes.
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