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Concept
Petrographic microscope
Summary
A petrographic microscope is a type of optical microscope used to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks. The method includes aspects of polarized light microscopy (PLM).
TOC
Depending on the grade of observation required, petrological microscopes are derived from conventional brightfield microscopes of similar basic capabilities by:
Adding a Nicol prism polarizer filter to the light path beneath the sample slide
Replacing the normal stage with a circular rotating stage (typically graduated with vernier scales for reading orientations to better than 1 degree of arc)
Adding a second rotatable and removable Nicol prism filter, called the analyzer, to the light path between objective and eyepiece
Adding a phase telescope, also known as a Bertrand lens, which allows the viewer to see conoscopic interference patterns
Adding a slot for insertion of wave plates
Petrographic microscopes are constructed with optical parts that do not add unwanted polarizing effects due to strained glass, or polarization by reflection in prisms and mirrors. These special parts add to the cost and complexity of the microscope. However, a "simple polarizing" microscope is easily made by adding inexpensive polarizing filters to a standard biological microscope, often with one in a filter holder beneath the condenser, and a second inserted beneath the head or eyepiece. These can be sufficient for many non-quantitative purposes.
The two Nicol prisms (occasionally referred to as nicols) of the petrographic microscope have their polarizing planes oriented perpendicular to one another. When only an isotropic material such as air, water, or glass exists between the filters, all light is blocked, but most crystalline materials and minerals change the polarizing light directions, allowing some of the altered light to pass through the analyzer to the viewer.
Official source
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