polarization(redirected from polarisation)
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polarization(poh-lă-ri-zay -shŏn) The degree to which the orientation of the electric or magnetic vector in electromagnetic radiation is predictable over time. In a partially polarized wave, the vector thus shows a less random orientation in the plane perpendicular to the direction of wave motion than occurs in an unpolarized (normal) wave. Waves in which the electric vectors are entirely vertical or horizontal with respect to the direction of motion are described as vertically or horizontally plane (or linearly) polarized. In general, both polarizations are present with a relative phase difference and the wave is then elliptically polarized in the right-handed or left-handed sense accordingly as the resultant vector rotates clockwise or counterclockwise when viewed along the direction of motion of the wave; in the case where the resultant vector rotates and its magnitude remains constant, the wave is circularly polarized.
Polarization is a measure of the way in which light or other electromagnetic radiation from a celestial body is affected by factors such as scattering due to cosmic dust or strong stellar or interstellar magnetic fields, or reflection from a surface. Radio emissions from celestial sources are usually partially polarized, i.e. the waves can be considered to be composed of a completely unpolarized component plus a small polarized component. Synchrotron emission, however, may be strongly polarized. The general situation is described by the four Stokes parameters (I, Q, U, and V ), which are defined in such a manner that specifying their four values uniquely describes the state of polarization: I is a measure of the total power in the wave, Q and U define the degree of linear polarization, and V that of circular polarization. All four have dimensions of flux density. See also Faraday rotation; interstellar polarization.