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.

## Definition

The wave impedance is given by

- $Z={E_{0}^{-}(x) \over H_{0}^{-}(x)}$

where $E_{0}^{-}(x)$ is the electric field and $H_{0}^{-}(x)$ is the magnetic field, in phasor representation. The impedance is, in general, a complex number.

In terms of the parameters of an electromagnetic wave and the medium it travels through, the wave impedance is given by

- $Z={E_{0}^{-}(x) \over H_{0}^{-}(x)}$