Optical medium

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.

The optical medium has an intrinsic impedance, given by

η = E x H y {\displaystyle \eta ={E_{x} \over H_{y}}}

where E x {\displaystyle E_{x}} and H y {\displaystyle H_{y}} are the electric field and magnetic field, respectively. In a region with no electrical conductivity, the expression simplifies to:

η = μ ε   . {\displaystyle \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 = μ 0 ε 0   . {\displaystyle Z_{0}={\sqrt {\mu _{0} \over \varepsilon _{0}}}\ .}

Waves propagate through a medium with velocity c w = ν λ {\displaystyle c_{w}=\nu \lambda } , where ν {\displaystyle \nu } is the frequency and λ {\displaystyle \lambda } is the wavelength of the electromagnetic waves. This equation also may be put in the form

c w = ω k   , {\displaystyle c_{w}={\omega \over k}\ ,}

where ω {\displaystyle \omega } is the angular frequency of the wave and k {\displaystyle k} is the wavenumber of the wave. In electrical engineering, the symbol β {\displaystyle \beta } , called the phase constant, is often used instead of k {\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]

c 0 = 1 ε 0 μ 0   , {\displaystyle c_{0}={1 \over {\sqrt {\varepsilon _{0}\mu _{0}}}}\ ,}

where ε 0 {\displaystyle \varepsilon _{0}} is the electric constant and   μ 0   {\displaystyle ~\mu _{0}\ } is the magnetic constant.

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

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