The Conférence Générale des Poids et Mésures (1983) defined the meter to be "the length of the path travelled by light in a vacuum during a time interval of 1/299792458 of a second". This sets the speed of light to be c = 299792458 m s−1 exactly.
The simplest propagating sinusoidal wave may be written in the form A = A0 cos (kx − ωt), where A0 is the amplitude. The term in the bracket is the phase of the wave, x and t being position and time. Thus, a surface of constant phase propagates with the phase velocity v = ω/k, where ω = 2πf is the angular frequency for frequency f and k = 2π/λ is the Wave Number for wavelength λ. In a vacuum v = c. Otherwise v = c/m, where m is the Refractive Index. The phase velocity may be measured by Interferometry.
Born, M. and Wolf, E. (1980) Principles of Optics, 6th edn., Pergamon Press, Oxford.