Chemiluminescence is the emission of light and heat as the result of a chemical reaction. It is a form of electromagnetic radiation which just happens to fall in the visible spectrum, making it easy to see and identify. The best-known example of chemiluminescence is probably the chemical reaction used to make glow sticks light up, which is the result of cracking a small vial of one chemical into a chemical it will react with.
When chemiluminescence occurs in animals, it is known as bioluminescence. Several animal species use this process as a signal, or to lure prey; fireflies are notable producers of chemiluminescent reactions, as are some algae. When subjecting chemicals to an electrical current creates a luminescent reaction, it is termed electrochemiluminescence.
There are all sorts of interesting applications for chemiluminescence. For example, Luminol, a chemical widely used in the forensics community, reacts with iron to generate a luminescent reaction. When forensics teams want to determine whether or not traces of blood are at a crime scene, they can spray the site with Luminol to generate a reaction. It can also be used to check for traces of other reactants at a site, assuming that a chemical which generates a chemiluminescent reaction can be obtained. Some gas phase reactions also exhibit this property, so chemiluminescence can be used to check for traces of certain impurities in the air.
The light from such reactions is often a dull green or blue in color, and the brightness varies, depending on the chemicals involved and the amount of chemicals present. Sometimes, chemiluminescence can be extremely bright, and such reactions are often used to demonstrate this interesting property in the lab to students. At other times, it may be much more subtle, requiring a sharp eye to detect the traces of the reaction.
As the reaction progresses, the chemical composition changes, and the light will begin to die off before fading away entirely as a result of conversion. Often, chemiluminescence is very brief, at least to the naked eye, although scientific tools can be used to measure the reaction as it continues. The stages of the reaction can also be used to provide clues, with researchers analyzing how long the reaction takes place at different phases, and how the color and brightness levels change.