Optical medium
# Optical medium

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 $c_0:$

    $c_{0}={1 \over {\sqrt {\varepsilon _{0}\mu _{0}}}}\ ,$
    where $\varepsilon _{0}$ is the electric constant and $~\mu _{0}\$ is the magnetic constant.

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

## Types

* Homogeneous medium vs. heterogeneous medium
* Transparent medium vs. opaque body
   * Translucent medium
