E = pc .
(We now know this to be the energy-momentum relationship for any massless particle.) The photon equation may therefore be rewritten in terms of photon momentum:
pc = hf .
Since c = f λ for a wave moving at speed c, this equation in turn can be written p = h/λ, identical to the de Broglie equation. This equivalence is true only for photons (or other massless particles). De Broglie’s contribution was the suggestion that the momentum-wavelength relationship should be true for all particles.