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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

$$\eta = {E_x \over H_y}$$

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 Z0, 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 c0:[1]

$$c_0 = {1 \over \sqrt{\varepsilon_0 \mu_0}}\ ,$$
where $$\varepsilon _{0}$$ is the electric constant and $$~ \mu_0 \$$ is the magnetic constant.

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

Homogeneous medium
Heterogeneous medium
Transparent medium
Translucent medium
Opaque body

Notes and references

With ISO 31-5, NIST and the BIPM have adopted the notation c0.
Raymond Serway & Jewett J (2003). Physics for scientists and engineers (6th ed.). Belmont CA: Thomson-Brooks/Cole. ISBN 0-534-40842-7.

Electromagnetic spectrum
Optics
SI units
Free space
Metamaterial
Photonic crystal
Photonic crystal fiber

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