A polarizing microscope is a microscope in which the object lies in the light path between two polarizing filters. It is thus illuminated or irradiated with polarized light, and the optical system of the microscope also includes a rotatable polarization filter.
If the object itself rotates the polarization direction of the light, this can be observed by the microscopist. He puts the two polarization filters in a mutually crossed state. Then normally no more light will be transmitted from the light source to the eye. Unless the object itself is also optically active, it will light up clearly, sometimes in striking rainbow colors.
Especially important in distinguishing different crystals and minerals.