What is the relationship between photon and electromagnetic wave? I am trying to understand light, it is so weird. Light is not an electromagnetic wave, since it has different speed in different mediums. Light does not have mass (the mass that can be derived from the energy is not a real mass), so its not a particle either. What is it? Back to my original question about EM wave being a matter. Well, it has energy, it has speed of distribution, it exerts pressure on reflecting or absorbing particles, so it must have a mechanical momentum, if it has momentum, then it must have mass. So it has all the components necessary to be considered matter. But its speed in any medium is the speed of light. But you cannot say the same thing about light, its speed changes with change in refractive index of the medium. Ok, you are saying that electromagnetic wave is a lot of photons. But the waves' speed changes as the refractive index of the medium changes. Yet photons can only exist while moving with the speed of light. Maybe that is where some of the photons are absorbed by the medium (or reflected). Yet the light still exists (because some photons got through the medium without change in speed?), only with smaller intensity? But then I don't understand. EM wave moves in any medium with a speed of light, no matter what the refractive index is. Then why doesn't the EM wave of light disappear when some photons are absorbed? I guess the question would be: WHy is any light absorbed at all if EM wave is supposed to travel at a speed of light through all mediums? It means that light is not EM at all! my head hurts. Or does it mean that light is light and EM wave is EM wave. And classical EM wave does not have any photons?