Editing Optical medium (section)

==Properties==
The optical medium has an ''[[intrinsic impedance]]'', given by
::<math>\eta = {E_x \over H_y}</math>
where <math>E_x</math> and <math>H_y</math> are the [[electric field]] and [[magnetic field]], respectively.
In a region with no [[electrical conductivity]], the expression simplifies to:

::<math>\eta = \sqrt{\mu \over \varepsilon}\ .</math>

For example, in [[free space]] the intrinsic impedance is called the [[characteristic impedance of vacuum]], denoted ''Z''<sub>0</sub>, and

::<math>Z_0 = \sqrt{\mu_0 \over \varepsilon_0}\ .</math>

Waves propagate through a medium with velocity <math>c_w = \nu \lambda </math>, where <math>\nu</math>  is the [[frequency]] and <math>\lambda</math> is the [[wavelength]] of the electromagnetic waves. This equation also may be put in the form
:<math> c_w = {\omega \over k}\ ,</math>
where <math>\omega</math> is the [[angular frequency]] of the wave and <math>k</math> is the [[wavenumber]] of the wave. In [[electrical engineering]], the symbol <math>\beta</math>, called the ''[[phase constant]]'', is often used instead of <math>k</math>.

The propagation velocity of electromagnetic waves in [[free space]], an idealized standard reference state (like [[absolute zero]] for temperature), is conventionally denoted by ''c''<sub>0</sub>:<ref>With [[ISO 31-5]], [[NIST]] and the [[BIPM]] have adopted the notation ''c''<sub>0</sub>.</ref> 
:<math>c_0 = {1 \over \sqrt{\varepsilon_0 \mu_0}}\ ,</math>
:where <math>\varepsilon_0</math> is the [[electric constant]] and <math>~ \mu_0 \ </math> is the [[magnetic constant]].

For a general introduction, see Serway<ref name=Serway>
{{cite book |author1       = Raymond Serway
 |author2       = Jewett J
 |name-list-style = amp
 |title         = Physics for scientists and engineers
 |edition       = 6th
 |year          = 2003
 |publisher     = Thomson-Brooks/Cole
 |location      = Belmont CA
 |isbn          = 0-534-40842-7
 |url           = https://archive.org/details/physicssciengv2p00serw
 |url-access    = registration
}}
</ref> For a discussion of synthetic media, see Joannopoulus.<ref name=Joannopoulos>
{{cite book 
|author1=John D Joannopouluos |author2=Johnson SG |author3=Winn JN |author4=Meade RD |title=Photonic crystals : molding the flow of light
|edition=2nd
|year= 2008
|publisher=Princeton University Press
|location=Princeton NJ
|isbn=978-0-691-12456-8
|url=http://ab-initio.mit.edu/book/}}
</ref>