These are some of the very fundamental things about the wave/particle-duality, I do not understand: 1) Do photons have a mass? 2) Do particles have a frequency? [tex] E = hf [/tex], how do I interpret this? 3) [tex] E = mc^2[/tex] and [tex] E = hf[/tex]. I've seen these two equations combined into [tex] mc^2 = hf[/tex], indicating that a photon does have a mass. I can't see why the to equations are combined, the E's in the equations are about something different.