My book says that when a photon carrying a certain amount of energy hits an electron, that gets excited and goes on an higher energetic level, absorbing the energy of the photon. When it comes back to its main energetic level it emits another photon.
Is this second photon having the same frequency as the first one and therefore same energy? If so, why there are black lines on the absorption spectogram?
Is this second photon having a lower frequency and therefore less energy (and then a different colour on the spectogram)?