wave impedance

At a point in an electromagnetic wave, the ratio of the electric field strength to the magnetic field strength. Note 1: If the electric field strength is expressed in volts per meter and the magnetic field strength is expressed in ampere-turns per meter, the wave impedance will have the units of ohms. The wave impedance, Z, of an electromagnetic wave is given by where is the magnetic permeability and ε is the electric permittivity. For free space, these values are 4 × 10^-7 H/m (henries per meter) and (1/36) F/m (farads per meter), from which 120, i.e., 377, ohms is obtained. In dielectric materials, the wave impedance is 377/n, where n is the refractive index. Note 2: Although the ratio is called the wave impedance, it is also the impedance of the free space or the material medium.