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Optical medium

## Summary

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

${\displaystyle \eta ={E_{x} \over H_{y}}}$

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

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

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

${\displaystyle Z_{0}={\sqrt {\mu _{0} \over \varepsilon _{0}}}\ .}$

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

${\displaystyle c_{w}={\omega \over k}\ ,}$

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

The propagation velocity of electromagnetic waves in free space, an idealized standard reference state (like absolute zero for temperature), is conventionally denoted by c0:[1]

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

For a general introduction, see Serway[2] For a discussion of synthetic media, see Joannopoulus.[3]

## Types

1. Homogeneous medium vs. heterogeneous medium
2. Transparent medium vs. opaque body