In a beam of electromagnetic radiation the vectors of electric field E and magnetic field H are perpendicular to the direction of the light propagation. Because vectors E and H of electromagnetic wave are perpendicular also each other, the state of the light anisotropy in the direction perpendicular to the wave propagation can be described by any of these two vectors. Generally, the polarization direction is the direction of the electric field vector.

Light emitted by separate atoms and molecules is always polarized. Nevertheless, any macroscopic source of the light consists of huge number of such separate emitters and the direction of the electric field at any moment of the time is not predictable. Such light is called unpolarized or natural light. Using light polarizer (polarization filter) we can suppress the component of the light polarized in one direction and transmit only the component polarized in perpendicular direction. Behind the polarizer the light will be plane-polarized. In general case, the totally polarized light consists of two perpendicular plane-polarized components. Depending on the amplitude of these two waves and their relative phase, the combined electric vector traces out an ellipse and the wave is said to be elliptically polarized. Elliptical and plane polarization can be converted into each other by means of birefringent optical systems.

Animation shows the experiment when the Gaussian beam with linear polarization is incident onto the rotating polarizer. As a result the intensity of light spot on the screen behind the polarizer is varied harmonically depending on the angle between the polarization direction and polarizer angle.