A polarizing microscope is used to determine rocks and minerals in thin sections. It is a standard light microscope with a 360° rotating stage, stress-free objectives, a polarizing filter to produce polarized light, and a second polarizing filter, called the "analyzer", in the light path from the objective to the eyepiece.
Petrographic microscopes may also be equipped with a "Bertrand lens" that focuses and magnifies the interference pattern imaged on the back focal plane of the objective. Petrographic microscopes are usually equipped with filters whose crystals are specially aligned, such as a quartz wedge, a quarter-wave mica filter, and a half-lambda mica filter. These can be sandwiched between the polarizers to reveal the positive and negative birefringence and thus the order of the minerals.
Most crystalline materials and minerals change the direction of polarization of light, so some of the changed light passes through the analyzer to the eyepieces. With a polarizer, it is possible to view the sample in what is called "rectilinearly polarized" light. If two polarizers are used, the sample can be examined in so-called "cross-polarized" light.
