What is a Polarizing Microscope?

A polarizing microscope is a microscope that is mainly used in geological studies to study geological specimens. For this reason, it is also known as a petrographic microscope. It is used in other scientific fields such as medicine and biology as well.

Polarizing microscopes are built like regular optical microscope, but are fitted with some extra features. Unlike regular microscopes which use normal light, a polarizing microscope uses polarized light to study specimens. In polarized light, the light waves vibrate in one direction; in normal light, the light waves vibrate in random directions.

Polarized light cannot be seen by human eyes in normal circumstances. It can, however, be used in polarized light microscopy to highlight features 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 several refractive indices; isotropic materials, which includes 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 rays of differing velocities.

Geological specimens are pared or ground into thin sections for study. The specimen to be studied is placed on a slide on a rotatable specimen stage. The specimen is then illuminated by a light source under the specimen stage.

The light passes through a polarizing filter called the polarizer and then passes through the birefringent specimen. The polarizer is usually fixed in an east to west vibrational direction, but it can be rotated as required. There is one more polarizing filter called the analyzer. It is usually situated above the objectives and can be moved in and out of the optical path.

The objectives used in a polarizing microscope are required to be strain free. The eyepiece may have a cross wire graticule or a photomicrography graticule. The cross wire graticule makes it easier to center in on the view. The photomicrography graticule is helpful in selecting an area for capture on film.

Many polarizing microscopes have a Bertrand lens. It is situated between the eyepiece and the objective. A Bertrand lens helps in studying the back focal plane of the objective to find out interference figure.

Compensation and retardation plates may also be used in polarizing light microscopy to better observe optical path differences. These plates can be inserted in a slot in the eyepiece or slotted in a tube between the body and eyepiece tubes.