Pleochroism

Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light.

It was first made compound in the German term Pleochroismus by mineralogist Wilhelm Haidinger in 1854, in the journal Annalen der Physik und Chemie.

The direction of the electric field determines the polarization of light, and crystals will respond in different ways if this angle is changed.

If absorption of light varies with the angle relative to the optical axis in a crystal then pleochroism results.

For example, hypersthene, which has two optical axes, can have a red, yellow, or blue appearance when oriented in three different ways in three-dimensional space.

In such cases, a thin section of the mineral is used and examined under polarized transmitted light with a petrographic microscope.