Optical medium
In optics, an optical medium is material through which light and other electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it. Properties
The optical 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 Z0, 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 c0: 

where  is the electric constant and  is the magnetic constant. For a general introduction, see Serway For a discussion of synthetic media, see Joannopoulus. Types

 Homogeneous medium vs. heterogeneous medium
 Transparent medium vs. opaque body
 Translucent medium

See also
Čerenkov radiation
Electromagnetic spectrum
Electromagnetic radiation
Optics
SI units
Free space
Metamaterial
Photonic crystal
Photonic crystal fiber

Notes and references

Optics
Electric and magnetic fields in matter