Optical medium

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

\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 Z₀, 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 c₀:^[1]