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
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:
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 For a discussion of synthetic media, see Joannopoulus.
See also Čerenkov radiation Electromagnetic spectrum Electromagnetic radiation Optics SI units Free space Metamaterial Photonic crystal Photonic crystal fiber
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