on the direction from the transmitter and other factors in the transmitting antenna design. See Stokes parameters. The term "FM radio" above refers to FM broadcasting, not two-way radio (more properly called land mobile radio), which uses vertical polarization almost exclusively. Dichroism Circular dichroism (CD) is the differential absorption of left- and right-handed circularly polarized light. Circular dichroism is the basis of a form of spectroscopy that can be used to determine the optical isomerism and secondary structure of molecules. In general, this phenomenon will be exhibited in absorption bands of any optically active molecule. As a consequence, circular dichroism is exhibited by most biological molecules, because of the dextrorotary (e.g., some sugars) and levorotary (e.g., some amino acids) molecules they contain. Noteworthy as well is that a secondary structure will also impart a distinct CD to its respective molecules. Therefore, the alpha helix, beta sheet and random coil regions of proteins and the double helix of nucleic acids have CD spectral signatures representative of their structures. Also, under the right conditions, even non-chiral molecules will exhibit magnetic circular dichroism — that is, circular dichroism induced by a magnetic field. Luminescence Circularly polarized luminescence (CPL) can occur when either a luminophore or an ensemble of luminophores is chiral. The extent to which emissions are polarized is quantified in the same way it is for circular dichroism, in terms of the dissymmetry factor, also sometimes referred to as the anisotropy factor. This value is given by: where corresponds to the quantum yield of left-handed circularly polarized light, and to that of right-handed light. The maximum absolute value of gem, corresponding to purely left- or right-handed circular polarization, is therefore 2. Meanwhile, the smallest absolute value that gem can achieve, corresponding to linearly polarized or unpolarized light, is zero. Mathematical description The classical sinusoidal plane wave solution of the electromagnetic wave equation for the electric and magnetic fields is: where k is the wavenumber; is the angular frequency of the wave; is an orthogonal matrix whose columns span the transverse x-y plane; and is the speed of light. Here, is the amplitude of the field, and is the normalized Jones vector in the x-y plane. If is rotated by radians with respect to and the x amplitude equals the y amplitude, the wave is circularly polarized. The Jones vector is: where the plus sign indicates left circular polarization, and the minus sign