Hi, Before I phrase my question, I'll just go through what I know so no unnecessary comments happen. I've read that the speed of light slows down in a medium because when a photon hits an electron the electron absorbs the energy of the photon and re-emits another photon. I also know that when an electron absorbs the energy of the photon it moves up a shell/energy band around the nucleus (I mean shell as in the electron setup 2,8,8 etc) Furthermore I have read that the electron then moves down a shell, back to it's original shell. My questions are the following: 1)When an electron moves down a shell - is this when it re-emits a photon? 2) How is it possible that an electron can move up a shell? I thought that the shells could only contain a specific number of electrons, hence the shells. 3) Why are there shells anyway? I read somewhere else that the electrons are in a cloud around the atom, with no specific location and therefore why should they be in shells? 4) If the electron has just gained the energy, does it move down again because it is unstable and cannot maintain this new energy? If so, why don't other electrons follow?