A material that glows in the dark after being exposed to light contains molecules that absorb energy for long periods of time, then later release that energy as light. The phenomenon is "phosphorescence" and the materials are said to be "phosphorescent."

Electrons in a molecule are normally arranged in a state of the least possible energy. However, when a molecule is exposed to light it may absorb a particle of light, called a "photon." The photon rearranges the electrons to a highly energetic state. If the material is phosphorescent, the extra energy from the photon becomes trapped in the molecule. Eventually, the electrons shift to a lower energy state and the extra energy that had been trapped is released as a photon of light. As these higher-energy electrons release light to return to their original states, fewer and fewer are left to escape. That is why the glow continually diminishes until it fades into nothing.

Three types of substances can glow in the dark. The first is the phosphorescent material
that absorbs light and then gradually releases it, like the phosphorescent material we just described. The second type can emit light without any outside help. For example, the firefly has the rare chemical luciferin and the enzyme luciferase in its abdomen. These interact with oxygen to produce the glow. Radioactive materials are the third type. These emit high-energy rays that react with surrounding materials to produce light. Radium is a good example of a radioactive material that gives off light.

In 1602 Vincenzo Casciarolo, a cobbler by trade and dilettante alchemist, discovered "Bolognian Phosphorus" (barium sulphide) on Monte Paderno just outside of Bologna. It was this natural stone, subsequently referred to also as the "Bolognian Stone" or "Litheophosphorus", that became the first object of scientific study of luminescent phenomena. 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 Homberg's phosphorus (calcium chloride), John Canton's phosphorus (calcium sulphide) and Balduin's phosphorus (calcium nitrate).

Around 1852 the term "fluorescence" was coined. While phosphorescence refers to a material that absorbs light and releases it over a period of time, fluorescence refers to a material that absorbs light in one spectral color and immediately emits light in a different spectral color. If you've ever seen a mineral glow under an ultraviolet light (black light) then you've seen fluorescence.

Fluorescence in minerals is a selective transformation of incoming ultraviolet energy, which is invisible to the human eye, into emitted visible light. Mineral fluorescence was discovered near the end of the 19th Century, when mines were electrified. Sparks from knife switches and the mine "trolleys" emitted ultraviolet light, revealing the fluorescent quality of willemite, calcite and other minerals.

Some female fireflies do not have wings and look very much like flat larvae. These are called "glowworms." Most glowworms give off a greenish light, but the "railroad worm" also has a red headlight. Some frogs eat so many fireflies that they also glow. Microscopic single celled (unicellular) algae called Dinoflagellates also cause phosphorescence. These are present throughout the ocean, and emit light when they are mechanically disturbed, for example by a boat paddle, the arm of a swimmer or wave action. This leads to a chemical reaction within the organism, resulting in light production.

~Source: "Do Fish Drink Water?" by Bill McLain