Symbol Vp. The speed of propagation of a pure sine wave. Vp = λf, where λ is the wavelength and f is the frequency. The value of the phase speed depends on the nature of the medium through which it is travelling and may also depend on the mode of propagation. For electromagnetic waves travelling through space the phase speed c is given by c2 = 1/ε0μ0, where ε0 and μ0 are the electric constant and the magnetic constant respectively.