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
::<math>\eta = {E_x \over H_y}</math>
where <math>E_x</math> and <math>H_y</math> are the electric field and magnetic field, respectively.
In a region with no electrical conductivity, the expression simplifies to:
::<math>\eta = \sqrt{\mu \over \varepsilon}\ .</math>
For example, in free space the intrinsic impedance is called the characteristic impedance of vacuum, denoted Z<sub>0</sub>, and
::<math>Z_0 = \sqrt{\mu_0 \over \varepsilon_0}\ .</math>
Waves propagate through a medium with velocity <math>c_w = \nu \lambda </math>, where <math>\nu</math> is the frequency and <math>\lambda</math> is the wavelength of the electromagnetic waves. This equation also may be put in the form
:<math> c_w = {\omega \over k}\ ,</math>
where <math>\omega</math> is the angular frequency of the wave and <math>k</math> is the wavenumber of the wave. In electrical engineering, the symbol <math>\beta</math>, called the phase constant, is often used instead of <math>k</math>.
The propagation velocity of electromagnetic waves in free space, an idealized standard reference state (like absolute zero for temperature), is conventionally denoted by c<sub>0</sub>:
:<math>c_0 = {1 \over \sqrt{\varepsilon_0 \mu_0\ ,</math>
:where <math>\varepsilon_0</math> is the electric constant and <math>~ \mu_0 \ </math> is the magnetic constant.
For a general introduction, see Serway For a discussion of synthetic media, see Joannopoulus.
See also
- Čerenkov radiation
- Electromagnetic spectrum
- Electromagnetic radiation
- Optics
- SI units
- Free space
- Metamaterial
- Photonic crystal
- Photonic crystal fiber