polarimetry
Polarimetry is a technique for measuring the polarization state of electromagnetic radiation. Light with a preferred orientation of the electric field is called polarized. The degree of polarization (from 0 to 1) and the angle of orientation provide information about the radiation sources and the propagation medium. Linear polarization arises from scattering by fine dust, reflection; circular polarization arises when passing through a medium with optical activity or anisotropic magnetic fields. In astronomical observations, polarizers and modulators, often with an analyzer, are used to construct polarization maps.
History
Étienne Malus discovered polarization in 1808 while looking at light reflected from a window of the Luxembourg Palace through a crystal of Icelandic spar. Later, James Maxwell and others linked polarization to electromagnetic theory.
How it works
Just as a slot in a fence allows only a rope oscillating along the slot to pass, a polarizer (a special filter) transmits light with only one direction of oscillation. By the change in intensity after passing through a rotating analyzer, the degree of polarization is determined.
💡 Some scarab beetles see circular polarization of the sky, which helps them navigate even when the Sun is hidden by clouds.