),[2] of a transmission medium is the ratio of the speed at which a wavefront (of an electromagnetic signal, a radio signal, a light pulse in an optical fibre or a change of the electrical voltage on a copper wire) passes through the medium, to the speed of light in vacuum.
For optical signals, the velocity factor is the reciprocal of the refractive index.
In electrical cables, the velocity factor mainly depends on the insulating material (see table below).
In a general science and engineering context, these terms would be understood to mean a true speed or velocity in units of distance per time,[3] while velocity factor is used for the ratio.
Velocity factor is an important characteristic of communication media such as category 5 cables and radio transmission lines.
Some typical velocity factors for radio communications cables provided in handbooks and texts are given in |the following table:[8][9] VF equals the reciprocal of the square root of the dielectric constant (relative permittivity),
, of the material through which the signal passes: in the usual case where the relative permeability,
In the most general case: which includes unusual magnetic conducting materials, such as ferrite.