Polarising microscope
A polarising microscope uses polarised light to identify minerals in thin rock slices by their colours and how they change as the slide is rotated.
Ordinary light vibrates in all directions. A polarising microscope, also called a petrographic microscope, has one polarising filter below the sample and a second, the analyser, above it, set at right angles. With both in place, a condition called crossed polars, nothing gets through unless the sample changes the light. The stage rotates, so the slide can be turned while you watch.
Most minerals split light into two rays travelling at different speeds, a property called birefringence. Under crossed polars this produces interference colours that help identify minerals: olivine shows bright, vivid colours, while plagioclase shows greys. As the stage turns, crystals go dark at certain angles, called extinction. Glass and isotropic minerals stay dark at every angle. The same instrument reveals shock effects such as wavy extinction and mosaicism, used to assign shock stages.
A neat example is Shergotty, the martian meteorite that fell in India in 1865. Its plagioclase was turned into the glass maskelynite by shock, so those areas stay black under crossed polars while the pyroxene glows with colour. Schools and universities use these microscopes; samples must be standard 30-micrometre thin sections.
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