title: Optical medium
text: # 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.
  
  ## Properties
  
  The optical 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 $Z_0,$ 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 $c_0:$
  
      $c_{0}={1 \over {\sqrt {\varepsilon _{0}\mu _{0}}}}\ ,$
      where $\varepsilon _{0}$ is the electric constant and $~\mu _{0}\$ is the magnetic constant.
  
  For a general introduction, see Serway For a discussion of synthetic media, see Joannopoulus.
  
  ## Types
  
  * Homogeneous medium vs. heterogeneous medium
  * Transparent medium vs. opaque body
     * Translucent medium
  