Phosphorescence, << `fos` fuh REHS uhns, >> is the light certain substances give off when they absorb energy. The general term for the emission of light due to the absorption of energy is luminescence. The term phosphorescence refers to luminescence that continues after the energy source has been removed. It may last for seconds, hours, or even days. Luminescence that occurs only while a substance is exposed to energy is called fluorescence. See Fluorescence ; Luminescence .
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. The phosphorescence given off by living things is called bioluminescence (see Bioluminescence ).
For a substance to phosphoresce, the electrons in its atoms must absorb energy. This energy causes the electrons to become excited—that is, to jump to a higher energy level. 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.
Phosphorescent substances are used to create the images on computer and television screens. Tiny dots of phosphorescent material called phosphors coat the screens. They phosphoresce briefly when a beam of electrons strikes them. We see a continuous picture as the beam sweeps back and forth across the screen.
