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

$eta\; =\; \{E\_x\; over\; H\_y\}$

where $E\_x$ and $H\_y$ are the electric field and magnetic field, respectively. In a region with no electrical conductivity, the expression simplifies to:

$eta\; =\; sqrt\{mu\; over\; varepsilon\}\; .$

For example, in free space the intrinsic impedance is called the characteristic impedance of vacuum, denoted Z₀, and

$Z\_0\; =\; sqrt\{mu\_0\; over\; varepsilon\_0\}\; .$

Waves propagate through a medium with velocity $c\_w\; =\; nu\; lambda$, where $nu$ is the frequency and $lambda$ is the wavelength of the electromagnetic waves. This equation also may be put in the form

$c\_w\; =\; \{omega\; over\; k\}\; ,$

where $omega$ is the angular frequency of the wave and $k$ is the wavenumber of the wave. In electrical engineering, the symbol $beta$, called the phase constant, is often used instead of $k$.

The propagation velocity of electromagnetic waves in free space, an idealized standard reference state (like absolute zero for temperature), is conventionally denoted by c₀:

$c\_0\; =\; \{1\; over\; sqrt\{varepsilon\_0\; mu\_0\}\}\; ,$

where $varepsilon\_0$ is the electric constant and $~\; mu\_0$ is the magnetic constant.

For a general introduction, see Serway For a discussion of man-made media, see Joannopoulus.

Notes and references

See also

• Electromagnetic spectrum
• Electromagnetic radiation
• Optics
• SI units
• Free space
• Photonic crystal
• Photonic crystal fiber
• Čerenkov radiation