Because I understand it and didn't before, and because I'm thrilled about the information, I am going to explain something about the emission of light. Very simply, when an atom's energy is increased, its electrons move into an orbit with a larger radius. The electrons then jump back to their original orbit, and this results in a release of the energy that caused them to move into a larger orbit in the first place, in the form of a photon. A photon, you remember, has zero rest mass and is a particle of pure energy.