Polarization

Abstract

Only the energy density of the radiation field could be studied thus far, because the vector nature of the electromagnetic field has been ignored. From Maxwell’s equations, the electric field of electromagnetic radiation is perpendicular to the propagation direction. In the case of monochromatic waves, the tip of the field vector traces a well-defined figure in the transverse plane, usually an ellipse, a property referred to as polarization. Real radiation fields are usually unpolarized since, in general, the polarization ellipses of the different wave components that make up the radiation field are randomly oriented and the overall electric field vanishes (although the average of the squared values of the field amplitudes does not vanish and provides the radiative intensity). However, when a certain fraction of the wave components share a common polarization ellipse, the overall radiation field, too, is polarized. Accordingly, the detection of polarized radiation indicates the existence of an overall structure or correlation in the emitting region. The situation can be likened to thermal motions of gas particles. Each particle has a well defined velocity, but the overall velocity (though not the velocity square) averages out to zero. Only when the particles share a common motion does the mean velocity not average out to zero.