Recently, I was teaching an undergraduate chemistry lab and we did an experiment where copper (I) oxide was oxidized to make copper (II) oxide. Some of the students reported seeing a green flame and they asked me what it was.
When you heat copper ions, the energy "excites" electrons and pushes them into higher energy levels. Being in this excited state is not particularly stable, so the electron rapidly returns to the ground state and emits a photon (a particle of light) to release the energy it gained by being excited.
These photons are the green light you see from excited copper atoms returning to the ground state.
(published on 10/22/2007)