In a common kind of fluorescent tube, a high voltage tears some electrons away from mercury vapor atoms—it's a brief pulse, so that mostly certain electrons and their band of transition energies are involved. After the pulse, they are able to fall quickly back into their usual orbits, emitting photon energy within their band frequencies. If the vapor were excited by a continuous high voltage, if the electrons weren't allowed to fall back quickly to their "ground" states, more of them would simply collide with other atoms, dropping energy in less predictable, less desirable ways. The vapor would heat up, becoming eventually incandescent. If the voltage were too low, a more ragged group of transitions would take place, mostly leading to mercury atoms running into one another at higher speeds—heat, again.

Now it happens the useful emissions in mercury vapor lie in the invisible, ultraviolet range. The walls of the fluorescent tube, however, are covered with a powdered material (eg., zinc sulfide) that absorbs photons in this band and instantly re-emits a larger number of photons at a lower energy, mostly visible light.