In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude but its direction rotates at a constant rate in a plane perpendicular to the direction of the wave.
In electrodynamics the strength and direction of an electric field is defined by its electric field vector. In the case of a circularly polarized wave, as seen in the accompanying animation, the tip of the electric field vector, at a given point in space, describes a circle as time progresses. At any instant of time, the electric field vector of the wave indicates a point on a helix oriented along the direction of propagation. A circularly polarized wave can rotate in one of two possible senses: right circular polarization in which the electric field vector rotates in a right-hand sense with respect to the direction of propagation and left circular polarization in which the vector rotates in a left-hand sense.
Circular polarization is a limiting case of the more general condition of elliptical polarization. The other special case is the easier-to-understand linear polarization.
The phenomenon of polarization arises as a consequence of the fact that light behaves as a two-dimensional transverse wave.