Plane-polarized electromagnetic radiation is produced by certain radiant energy sources. For example, the radio waves emanating from an antenna and the microwaves produced by a klystron tube are both plane polarized. Visible and ultraviolet radiation from relaxation of a single excited atom or molecule is also polarized, but the beam from such a source has no net polarization since it is made up of a multitude of individual wave trains produced by an enormous number of individual atomic or molecular events. The plane of polarization of these individual waves is random so that their individual polarizations cancel.