Optical medium

For light-based digital storage media, see Optical disc.

In optics, an optical medium is material through which light and other electromagnetic waves propagate. It is a form of transmission medium. The permittivity and permeability of the medium define how electromagnetic waves propagate in it.

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 Z₀, 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 c₀:<ref>With ISO 31-5, NIST and the BIPM have adopted the notation c₀.</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> Raymond Serway & Jewett J (2003). Physics for scientists and engineers (6th ed.). Belmont CA: Thomson-Brooks/Cole. ISBN 0-534-40842-7. </ref> For a discussion of synthetic media, see Joannopoulus.<ref name=Joannopoulos> 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. </ref>

Types

1. Homogeneous medium vs. heterogeneous medium
2. Transparent medium vs. opaque body

• • Translucent medium

See also

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