Different Kinds of Luminescence

Energy is transferred in many different ways: as heat, as light, or by chemical reactions. When energy is released by matter in the form of light it is referred to as luminescence. An exception is usually made for matter that has such a high temperature that it simply glows; this is called incandescence. Luminescence is far more efficient than incandescence. It neither requires nor generates much heat, so it's sometimes known as “cold light” (from Aristotle’s term).

Chemiluminescence is the emission of light as the result of a chemical reaction. The energy that is released as a result of the chemical reaction excites the product molecules of the reaction. A molecule in this excited state either relaxes back to a lower energy state, with the direct emission of light, or transfers its energy to a second molecule, which becomes the light emitter.

A common example is a glowstick. When the glowstick is bent, chemicals are released and mix together, causing the glowstick to start glowing. Another example of a chemiluminescent reaction is between nitrogen monoxide (NO) and ozone (O₃) which forms NO₂ (nitrogen dioxide) in an excited state. The activated NO₂ emits light when it reverts back to a lower energy state.

Chemiluminescence differs from photoluminescence in that the excited state of the electron is derived from the product of a chemical reaction rather than the more typical way of creating electronic excited states, namely absorption of a quantum or multiple quanta of light. In photoluminescence, light is used to drive an endothermic chemical reaction. In chemiluminescence, light is generated from a chemically exothermic reaction. There are two types of photoluminescence: (i) fluorescence, which involves absorbing and releasing lower energy light almost immediately and (ii) phosphorescence, in which the release of light is delayed, making phosphorescent materials appear to glow in the dark.

Bioluminescence is a naturally occurring form of chemiluminescence, or the emission of visible light by an organism as a result of a natural chemical reaction. Bioluminescence is primarily a marine phenomenon, though bioluminescence can also be found in some land animals (predominantly insects), fungi, bacteria, and protists. It is the predominant source of light in the deep ocean. There are many reasons for this. First, large portions of the ocean either have very dim light or exist in total darkness. Second, the volume of habitat where bioluminescence is effective is vast, allowing natural selection to take place in a huge ecological context. Third, in most of the ocean there is no concealment; the most common functions of bioluminescence in the ocean are for defense against predators or to find or attract prey.