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:^[1]

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^[2] For a discussion of man-made media, see Joannopoulus.^[3]

Notes and references

1. ↑ With ISO 31-5, NIST and the BIPM have adopted the notation c₀.
2. ↑
3. ↑

See also

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

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