Linkage (mechanical)

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

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

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:

$\eta =\sqrt{\frac{\mu }{\epsilon }}.$

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

$Z_0=\sqrt{\frac{{\mu }_0}{{\epsilon }_0}}.$

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

$c_w=\frac{\omega }{k},$

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:

$c_0=\frac{1}{\sqrt{{\epsilon }_0{\mu }_0}},$

where ${\epsilon }_0$ is the electric constant and ${\mu }_0$ is the magnetic constant.

For a general introduction, see Serway For a discussion of man-made media, see Joannopoulus.

Notes and references

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See also

• Čerenkov radiation
• Electromagnetic spectrum
• Electromagnetic radiation
• Optics
• SI units
• Free space
• Metamaterial
• Photonic crystal
• Photonic crystal fiber