At a point in an electromagnetic wave , the ratio of the electric field strength to the magnetic field strength. ( 188 ) 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 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 .