Luminescence that occurs only while a substance is exposed to energy is called fluorescence. The energy to create phosphorescence may be supplied by various sources, including electric current, ultraviolet rays, X rays, and certain chemical reactions. The color of the phosphorescent light depends on the substance and the form of energy it absorbs. Familiar phosphorescent substances include Celluloid, eggshells, ivory, and paraffin. Many gems, minerals, and pigments phosphoresce strongly when excited by nuclear or ultraviolet radiations.
Some materials phosphoresce simply by being exposed to sunlight.
Manufacturers may use such materials in toys or the markings on watch faces that glow in the dark. For a substance to phosphoresce, the electrons in its atoms must absorb energy. Excited electrons are unstable. To drop back to their normal energy level, the electrons give off the excess energy as light. But in a phosphorescent substance, the electrons become trapped temporarily between the excited energy level and the normal level.