Editing Relative permittivity (section)

== Metals ==
Permittivity is typically associated with [[dielectric materials]], however metals are described as having an effective permittivity, with real relative permittivity equal to one.<ref name=Lourtioz>{{cite book |last=Lourtioz |first=J.-M. |url=https://books.google.com/books?id=vSszZ2WuG_IC&pg=PA121 |title=Photonic Crystals: Towards Nanoscale Photonic Devices |year=2005 |publisher=Springer |isbn=978-3-540-24431-8 |display-authors=etal |pages=121–122}} equation&nbsp;(4.6), page&nbsp;121</ref> In the high-frequency region, which extends from radio frequencies to the far [[infrared]] and [[Terahertz radiation|terahertz]] region, the plasma frequency of the electron gas is much greater than the electromagnetic propagation frequency, so the refractive index ''n'' of a metal is very nearly a purely imaginary number. In the low frequency regime, the effective relative permittivity is also almost purely imaginary: It has a very large imaginary value related to the conductivity and a comparatively insignificant real-value.<ref>Lourtioz (2005), equations&nbsp;(4.8)–(4.9), page&nbsp;122</ref>