Polarization (also polarisation) is a property of waves that describes the orientation of their oscillations. This article primarily covers the polarization of electromagnetic waves such as light, although other types of wave also exhibit polarization.

By convention, the polarization of light is described by specifying the direction of the wave's electric field. When light travels in free space, it propagates as a transverse wave—the polarization is perpendicular to the wave's direction of travel. In this case, the electric field may be oriented in a single direction (linear polarization), or it may rotate as the wave travels (circular or elliptical polarization). In the latter cases, the oscillations can rotate rightward or leftward in the direction of travel, and which of those two rotations is present in a wave is called the wave's chirality. In a waveguide such as an optical fiber, the description of the wave's polarization is more complicated, as the fields can have longitudinal as well as transverse components.

For longitudinal waves such as sound waves in fluids, the direction of oscillation is by definition along the direction of travel, so there is no polarization. In a solid medium, however, sound waves can be transverse. In this case, the polarization is associated with the direction of the shear stress in the plane perpendicular to the propagation direction. This is important in seismology.

Polarization is significant in areas of science and technology dealing with wave propagation, such as optics, seismology, and telecommunications. The polarization of light can be measured with a polarimeter.

Basics: plane waves

The simplest manifestation of polarization to visualize is that of a plane wave, which is a good approximation of most light waves (a plane wave is a wave with infinitely long and wide wavefronts). For plane waves the transverse condition requires that the electric and magnetic field be perpendicular to the direction of propagation and to each other. Conventionally, when considering polarization, the electric field vector is described and the magnetic field is ignored since it is perpendicular to the electric field and proportional to it. The electric field vector of a plane wave may be arbitrarily divided into two perpendicular components labeled x and y (with z indicating the direction of travel). For a simple harmonic wave, where the amplitude of the electric vector varies in a sinusoidal manner in time, the two components have exactly the same frequency. However, these components have two other defining characteristics that can differ. First, the two components may not have the same amplitude. Second, the two components may not have the same phase, that is they may not reach their maxima and minima at the same time. Mathematically, the electric field of a plane wave can be written as,

$\backslash vec\{E\}(\backslash vec\{r\},t)\; =\; \backslash mathrm\{Re\}\; \backslash leftA\_\{y\}\backslash cdot\; e^\{i\backslash phi\},\; 0\; \backslash right)\; e^\{i(kz\; -\; \backslash omega\; t)\}\; \backslash right$