The situation in a reflected- or transmitted-light microscope when the pieces of Polaroid of both the analyser and substage polarizer are inserted into the light path through the microscope. The pieces of Polaroid are aligned at right angles to each other so that no light is transmitted to the observer, but a mineral (or rock) thin section present on the microscope stage interferes with the light rays into which the light is split by the mineral and produces interference leading to interference colours. See also birefringence.