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. Phosphorescence serves as a valuable tool in the scientific examination of various materials. For example, physicians can diagnose certain diseases by studying the phosphorescent light given off by human tissue exposed to ultraviolet rays. Archaeologists can learn the age of pottery by heating the pottery and measuring the amount of phosphorescence that results.