Hi all! I've been thinking of something lately. When an atom absorbs an incoming photon, the atom must gain some momentum in order to conserve linear momentum, right? Sort of like a totally inelastic collision? This momentum corresponds to some amount of kinetic energy and thus a raise in temperature of the gas, liquid or bulk of which the atom is a part. This is how I've understood heating by irradiation, please correct me if I'm wrong. Now the atom, having absorbed a photon, is in an excited electronic state. By means of spontaneous emission it will again return to its ground state by emitting a photon. But what about the energy that is dissipated throughout the material? In a gas, for example, suppose the atom transfers some of its kinetic energy to another atom before emitting a photon and returning to its ground state, there seems to me to have somehow entered extra energy? Like this: photon energy in (E) = photon energy out (E) + heating of gas (Q) Since the photon energies are the same, where does Q come from? Surely I'm missing something in my very primitive analysis, but where do I go wrong?