Phosphorescence is delayed luminescence or "afterglow". When an electron is kicked into a high-energy state, it may get trapped there for some time. In some cases, the electrons escape the trap in time; in other cases they remain trapped until some trigger gets them unstuck. Many glow-in-the-dark products, especially toys for children, involve substances that receive energy from light, and emit the energy again as light later.

Phosphorescence is a name given to a variety of physical phenomena due to different causes, but all consisting in the emission of a pale, more or less ill-defined light, not obviously due to combustion. The word was first used by physicists to describe the property possessed by many substances of themselves becoming luminous after exposure to ultraviolet light. The first discovery of this property which apparently attracted scientific attention seems to have been that of the Bologna stone (barium sulphide), which was discovered by Vincenzo Cascariolo, a cobbler of Bologna, in about 1602. This was followed by the discovery of a number of other substances which become luminous either after exposure to light or on heating, or by attrition, and to which the general name of phosphori was given. Among these may be mentioned calcium chloride, calcium sulphide and calcium nitrate. Over the years it has been found convenient to limit the strict meaning of the word phosphorescence to the case of minerals which, after exposure to ultraviolet light, become self-luminous (even if only for a fraction of a second). The general term Luminescence includes all cases in which minerals give off light not due to ignition. This general term embraces several subdivisions. Thus, fluorescence (q.v.) and phosphorescence are included under the same heading, photoluminescence, being distinguished from each other only by the fact that fluorescent minerals emit their characteristic light only while under the influence of the exciting illumination, while phosphorescent minerals are luminous for an appreciable time after the exciting light is cut off.

As in the case of fluorescent minerals, the light produced by phosphorescent minerals consists commonly of rays less refrangible than those of the exciting light. Thus the ultraviolet portion of the spectrum is usually the most efficient in exciting rays belonging to the visible part of the spectrum. The phosphorescence of calcium suiphide and other phosphori depends on the presence of minute quantities of other substances, such as copper, bismuth and manganese. The maximum intensity of phosphorescent light is obtained when a certain definite proportion of the impurity is present, and the intensity is diminished if this proportion is increased.