Hello everyone, I've been studying the origins to quantum physics lately. It has been bugging me for a while that everywhere I look for information concerning the equation E=hf, does not explain where exactly the equation comes from. I know Plancks constant comes from the explanation Planck gave to the black body radiation problem. But, how exactly did the equation for the photon, E=hf, appear? I have another question if you don't mind answering this as well: Given Einstein's equation for total energy E^2 = p^2c^2 + m^2c^4 ... if you want to get the energy for a photon, you have that m=0 so the energy is E=pc. Now, using E=hf, we get c/f = h/p , so: L=h/p , where L is the wavelength. Then De Broglie says that this is the wavelength for ALL particles. But how can one say this if we used equations for energy that ONLY concern the photon. How can you use the same equation to describe all particles if we made m=0 in the process???