The
wave impedance of an
electromagnetic wave is the
ratio of the transverse components of the
electric and
magnetic fields (the transverse components being those at right angles to the direction of propagation). For a transverse-electric-magnetic (
TEM)
plane wave traveling through a homogeneous
medium, the wave impedance is everywhere equal to the intrinsic impedance of the medium. In particular, for a plane wave travelling through empty space, the wave impedance is equal to the
impedance of free space. The symbol
Z is used to represent it and it is expressed in units of
ohms. The symbol η (
eta) may be used instead of
Z for wave impedance to avoid confusion with
electrical impedance.