From AMS Glossary
- With respect to a transverse electromagnetic wave, the correlation between two orthogonal components of its electric (or, equivalently, magnetic) field.
If the ratio of the amplitudes of these two components and the difference in their phases is constant in time (completely correlated), the wave is said to be polarized (or completely polarized or 100% polarized). If these two amplitudes and phases are uncorrelated, the wave is said to be unpolarized (or 0% polarized). These are two extreme degrees of correlation, never strictly realized in nature, all real waves being partially polarized (or partially correlated). Associated with a polarized wave is its vibration ellipse traced out in time by the oscillating electric field at a given point in space. A line (circle) is a special ellipse, and a wave with such a vibration ellipse is said to be linearly (circularly) polarized, but the general state of (complete) polarization is elliptical. A vibration is characterized by its handedness (the sense in which it rotates; clockwise or counterclockwise), the ratio of its minor to major axis (ellipticity), and its orientation (azimuth). Any beam may be decomposed uniquely as an incoherent superposition of two beams, one unpolarized and one polarized. Thus, the ratio of the transmitted power of the polarized component to the total transmitted power may be taken as a measure of the degree of polarization of the beam. The vibration ellipse of the polarized component and the degree of polarization define the state of polarization of the beam. Polarization would be an uninteresting (indeed, unmeasurable) property of electromagnetic radiation were it not for the fact that two beams, identical in all respects except their state of polarization, may interact with matter differently. Skylight (for a molecular atmosphere) is, in general, partially linearly polarized, the degree of polarization being greatest approximately 90° from the sun.