Characteristic impedance of vacuum
The impedance of free space, Z0, is a physical constant relating the magnitudes of the electric and magnetic fields of electromagnetic radiation travelling through free space. That is, Z0 = |E| / |H|, where |E| is the electric field strength and |H| is the magnetic field strength. It has an exactly defined value
The impedance of free space (more correctly, the wave-impedance of a plane wave in free space) equals the product of the vacuum permeability μ0 and the speed of light in vacuum c0. Since the values of these constants are exact (they are given in the definitions of the ampere and the metre respectively), the value of the impedance of free space is likewise exact.
The analogous quantity for a plane wave travelling through a dielectric medium is called the intrinsic impedance of the medium, and designated η (eta). Hence Z0 is sometimes referred to as the intrinsic impedance of free space, and given the symbol η0. It has numerous other synonyms, including:
From the above definition, and the plane wave solution to Maxwell's equations,
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