Electromagnetic (EM) radiation (light) is a mutually perpendicular oscillating electric (E) and magnetic (M) fields in a plane transverse to the direction of propagation (see an example in the figure below). Light polarization is a macroscopic manifestation of the vectorial nature of the E-field.

Natural light is usually unpolarized and can be thought of as containing, for example, linear oscillating E-fields distributed equally over all possible directions. Unpolarized light can be polarized by means of reflection, refraction or absorption. Polaroid absorbing polarizers (in a dark sheet format) are frequently used for such purposes. Note, however, that the absorption entails a loss of at least 50% of the light.

In a linearly polarized light beam the oscillation direction of the E-field is the same (up to a ± sign) everywhere. At a given point in space one can imagine the E-field oscillating along a line (the polarization direction) with a varying length: from +E to -E and vice versa. The following drawing depicts schematically a snapshot (time is "frozen") of a linearly polarized beam (in the Y-direction) propagating along the +Z direction.

The EM-field is transverse: the E-field oscillates in the xy-plane, which is perpendicular to the propagation direction (z-axis in the drawing above). A general E-field at a given point in space is a vector in the xy-plane, which over time can change direction and magnitude. Such a vector can be describes as a superposition of two linear vectors (the basis) like (Ex, Ey), the components along the x-axis and y-axis respectively. Such description uses two linearly polarized electric fields to describe a general E-field. By varying the relative length of these components as well as their relative phase one can generate all possible polarization states.

Another useful and equivalent orthogonal basis exist: left-hand and right-hand circular polarizations. Thus, a general E-field can be described by a variety of polarization bases. The particular choice is usually dictated by the physical problem: light propagation in isotropic materials (glass), in uniaxial crystals (quartz, Lithium Niobate) or in biaxial crystals (mica) is easiest to describe in terms of the linearly polarized components. On the other hand, light propagation through magnetic or optically active materials is easier to describe in terms of circularly polarized components.

In a configuration where a circularly polarized light propagates towards an observer, the E-field at a fixed point in space would be seen to trace a circle. If the E-field moves in time counter-clockwise the polarization is defined as right-handed (RH) circular. It is a left-handed (LH) circular polarization if the E-field rotates clockwise. The two polarization states are depicted in the following figure.

Left-handed (on the left) and right-handed (on the right)
circular polarization