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

η = E x H y {\displaystyle \eta ={E_{x} \over H_{y}}}

where E x {\displaystyle E_{x}} and H y {\displaystyle H_{y}} are the electric field and magnetic field, respectively. In a region with no electrical conductivity, the expression simplifies to:

η = μ ε . {\displaystyle \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 = μ 0 ε 0 . {\displaystyle Z_{0}={\sqrt {\mu _{0} \over \varepsilon _{0}}}\ .}

Waves propagate through a medium with velocity c w = ν λ {\displaystyle c_{w}=\nu \lambda }, where ν {\displaystyle \nu } is the frequency and λ {\displaystyle \lambda } is the wavelength of the electromagnetic waves. This equation also may be put in the form

c w = ω k , {\displaystyle c_{w}={\omega \over k}\ ,}

where ω {\displaystyle \omega } is the angular frequency of the wave and k {\displaystyle k} is the wavenumber of the wave. In electrical engineering, the symbol β {\displaystyle \beta }, called the phase constant, is often used instead of k {\displaystyle 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 ε 0 μ 0 , {\displaystyle c_{0}={1 \over {\sqrt {\varepsilon _{0}\mu _{0}}}}\ ,}

where ε 0 {\displaystyle \varepsilon _{0}} is the electric constant and μ 0 {\displaystyle ~\mu _{0}\ } is the magnetic constant.

For a general introduction, see Serway For a discussion of synthetic media, see Joannopoulus.

See also

Notes and references