Because electromagnetic waves are transverse, their electric field can oscillate in different directions perpendicular to propagation. Polarization describes this orientation and its evolution in time.

Linear polarization

A wave is linearly polarized when the electric-field vector oscillates along one fixed transverse direction.

An ideal linear polarizer transmits the component of the electric field along its transmission axis.

Malus's law

If linearly polarized light of intensity $I_0$ enters an ideal polarizer whose axis makes angle $\theta$ with the polarization direction, the transmitted intensity is

$$I=I_0\cos^2\theta.$$

Circular and elliptical polarization

Two perpendicular field components with an appropriate phase difference can make the electric-field tip rotate rather than remain on one line. Equal components in quadrature produce circular polarization; more general combinations produce elliptical polarization.

Unpolarized light

In unpolarized light, the transverse electric-field direction varies in an effectively random way over the observation time.

Polarization provides direct evidence that electromagnetic waves are transverse and is used in optics, antennas, displays, imaging and communication systems.