An optical medium is material through which "electromagnetic waves propagate. It is a form of "transmission medium. The "permittivity and "permeability of the medium define how electromagnetic waves propagate in it. The medium has an "intrinsic impedance, given by
where and are the "electric field and "magnetic field, respectively. In a region with no "electrical conductivity, the expression simplifies to:
For example, in "free space the intrinsic impedance is called the "characteristic impedance of vacuum, denoted Z_{0}, and
Waves propagate through a medium with velocity , where is the "frequency and is the "wavelength of the electromagnetic waves. This equation also may be put in the form
where is the "angular frequency of the wave and is the "wavenumber of the wave. In "electrical engineering, the symbol , called the "phase constant, is often used instead of .
The propagation velocity of electromagnetic waves in "free space, an idealized standard reference state (like "absolute zero for temperature), is conventionally denoted by c_{0}:^{[1]}
For a general introduction, see Serway^{[2]} For a discussion of man-made media, see Joannopoulus.^{[3]}