What is fluorescence?

   Fluorescence is a process in which light is emitted by atoms and molecules that have been struck by ultraviolet rays, X-rays, gamma rays, or atomic particles. The light is emitted almost instantly and does not continue after the source of rays or atomic particles is removed.

   When radiation is absorbed by an atom or molecule, the energy of the atom's or molecule's electrons is increased. The atom or molecule is in an unusual, excited state. Some of the absorbed energy is lost by collision with other atoms or molecules, and the particle cannot maintain itself in its excited state. The electrons give up their remaining unusual energy and return to their original state. The energy given up is emitted from the atom or molecule as light. The light given off is radiation of a longer wavelength than the radiation received.

   The image on a television is the result of fluorescence induced by electron bombardment. Scintillation counters, used to detect radioactivity, respond to high-energy particles or radiation because such particles cause the scintillator crystal to fluoresce.