Medium (optics)

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

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

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

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

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