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
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: :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. ==See also==