Image: J.E. LLOYD
To explain this phenomenon, we might first break down its name and look at the meaning of its pieces. The first, chemi, means that it has to do with chemicals, and the second, luminescence, that it gives off light. Put together then, chemiluminescence means giving off light via a chemical reaction. To fully understand this definition, though, it is useful to back up and ask what causes the luminescence with which we are most familiar: the light from a lightbulb.
In an incandescent lightbulb, an electric current is passed through a filament, or thin metal wire. Because there is some resistance to the current flow, the filament gets quite hot, causing the metal's electrons to become "excited," or enter a higher energy state. When the electrons relax to their normal, or ground, state, they release this excess energy in the form of light. But in this particular process, the metal remains a metal; it does not undergo a chemical change.
A chemical change, on the other hand, occurs when a molecule's bonds are actually altered. For example, the reaction between hydrogen (H2) and oxygen (O2) to form water (H2O) is an example of a chemical change, because the H-H bond in H2 and the O-O bond in O2 are broken when new H-O bonds are formed to make H2O. For the most part, when chemicals undergo change in this way, the reactions either give off (exothermic) or absorb (endothermic) heat. The H2 plus O2 reaction is exothermic.
That said, there are a few very intriguing kinds of chemical reactions in which the energy produced is given off not as heat but as light. These reactions are what we term chemiluminescent, or in living organisms, bioluminescent. The most familiar terrestrial example of this "cold light" takes place in the common firefly. In the firefly, an enzyme called luciferase (a name meaning "light-bearing") triggers a reaction that produces energy emitted as light--the flashing beacon from the insect's lower abdomen.
Chemiluminescence is also found in some fungi and earthworms. It is most common, however, in the oceans, where many organisms, from fish to worms living at great depths, have glowing organs. Chemists have exploited these light-emitting reactions as markers in a large number of laboratory and clinical tests. The same reaction produces the light from emergency "light sticks" sold to campers and the glowing necklaces seen at concerts and sporting events.