Fluorescence

Some materials continue to emit light for a time after the incident radiation has been cut off. This is phosphorescence, usually a property of crystals or of large organic molecules. Phosphorescence often depends on the presence of minute quantities of impurities or imperfections in the crystal that provide "traps" for excited electrons. These electrons have received extra energy from incident radiation. The electrons remain in the "traps" until shaken loose by the heat vibrations of the atoms in the crystal. Phosphorescent light is emitted as the electrons return to their normal positions. Solid substances that produce light in this way are called phosphors.