Free space is the most simple and elementary electromagnetic medium. A medium is the stuff that waves propagate through. The properties, permittivity and permeability, define how electromagnetic waves travel through a medium. Also, the medium has an intrinsic impedance:

where μ is the permeability of the medium and is the permittivity of the medium.

Waves propagate through a medium with velocity of c_w. Simply the propagation velocity of a wave in a given medium is just the wavelength multiplied by its frequency, or its the angular frequency divided by the wavenumber(physics) or phase constant(electrical engineering).

The propagation velocity of electromagnetic waves in free space (medium) is

where, is the vacuum permittivity of free space

is the vacuum permeability of free space.

See also: Electromagnetic spectrum, Electromagnetic radiation, Optics.