Medium through which electromagnetic waves propagate

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

- $\eta ={E_{x} \over H_{y))$

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 *Z*_{0}, 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 *c*_{0}:^{[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 synthetic media, see Joannopoulus.^{[3]}