The velocity factor (VF),[1] also called wave propagation speed or velocity of propagation (VoP or v P {\displaystyle v_{\mathrm {P} }} ),[2] of a transmission medium is the ratio of the speed at which a wavefront (of an acoustic signal, for example, or an electromagnetic signal, a radio signal, a light pulse in a fibre channel or a change of the electrical voltage on a copper wire) passes through the medium, to the speed of light in a vacuum. For optical signals, the velocity factor is the reciprocal of the refractive index.