I know that in vacuum light travels with the speed c. But in different materials(eg water) the spedd is lower than c. I wanted to know what's happening that the speed decreases, and didn't came to a conclusion. given the energy of the photon ε=hf=√(m_0²c⁴ + p²c²)=pc=M·v·c since the rest mass m_0 is null, in a material the speed v it is equal to hf/Mc. However, M·v·c must be mc²(where M is the mass of the photon travelling the material, and m is the mass in vacuum), so, if the speed decreases the mass must increase, or the frequency must decrease(this is not possible, we're not speaking about the Compton effect, and the frequency of light characterize the source emitting the light, not the medium in which it travels) If the mass must increase, than by extrapolation, the rest mass must be ∞,which is not true. So my aproach of the problem is wrong, it must be. The speed of the photons in every material must be c, so in a material it must travel a greater distance than in vacuum. I mean, the photons could diffract multiple times, so this is causing them to travel a bigger distance, hence the slower light speed in materials. If diffraction is not the case, I cannot see why the speed of light should decrease. Excuse my bad english.