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An optical medium is material through which "electromagnetic waves propagate. It is a form of "transmission medium. The "permittivity and "permeability of the medium define how electromagnetic waves propagate in it. The 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 man-made media, see Joannopoulus.[3] Types of optical mediums

1 Homogenous medium 2 Heterogeneous medium 3 Transparent medium 4 Translucent medium 5 Opaque body

## Notes and references

1. ^ With "ISO 31-5, "NIST and the "BIPM have adopted the notation c0.
2. ^ Raymond Serway & Jewett J (2003). Physics for scientists and engineers (6th ed.). Belmont CA: Thomson-Brooks/Cole. "ISBN "0-534-40842-7.
3. ^ John D Joannopouluos; Johnson SG; Winn JN; Meade RD (2008). Photonic crystals : molding the flow of light (2nd ed.). Princeton NJ: Princeton University Press. "ISBN "978-0-691-12456-8.