Polarization, also called wave polarization, is an expression of the orientation of the lines of electric flux in an electromagnetic field (EM field). Polarization can be constant -- that is, existing in a particular orientation at all times, or it can rotate with each wave cycle.
Polarization is important in wireless communications systems. The physical orientation of a wireless antenna corresponds to the polarization of the radio waves received or transmitted by that antenna. Thus, a vertical antenna receives and emits vertically polarized waves, and a horizontal antenna receives or emits horizontally polarized waves. The best short-range communications is obtained when the transmitting and receiving (source and destination) antennas have the same polarization. The least efficient short-range communications usually take place when the two antennas are at right angles (for example, one horizontal and one vertical). Over long distances, the atmosphere can cause the polarization of a radio wave to fluctuate, so the distinction between horizontal and vertical becomes less significant.
Some wireless antennas transmit and receive EM waves whose polarization rotates 360 degrees with each complete wave cycle. This type of polarization called elliptical or circular polarization can be either clockwise or counterclockwise. The best communications results are obtained when the transmitting and receiving antennas have the same sense of polarization (both clockwise or both counterclockwise). The worst communications usually take place when the two antennas radiate and receive in the opposite sense (one clockwise and the other counterclockwise).
Polarization affects the propagation of EM fields at infrared (IR), visible, ultraviolet (UV), and even X-ray wavelengths. In ordinary visible light, there are numerous wave components at random polarization angles. When such light is passed through a special filter, the filter blocks all light except that having a certain polarization. When two polarizing filters are placed so a ray of light passes through them both, the amount of light transmitted depends on the angle of the polarizing filters with respect to each other. The lightest is transmitted when the two filters are oriented so they polarize light in the same direction. The least light is transmitted when the filters are oriented at right angles to each other.
The effect of polarization on visible light can be striking. Anyone who has worn polarized sunglasses, or who has used polarizing filters in photography, knows how a clear sky polarizes sunlight. Polarized sunglasses can reduce glare reflected from surfaces; this is useful under certain driving conditions and can also make it easier to see beneath the surface of a body of water. In twisted nematic displays (TN displays), polarizing filters are used in conjunction with a special liquid to brighten and darken regions of the display as external voltages are applied. This makes it possible to display alphanumeric characters in wristwatches, cell phones, and various other consumer electronic devices.