Luminescence, << `loo` muh NEHS uhns, >> is the giving off of light by means other than heat. The emission of light by means of heat is known as incandescence. Luminescence usually involves visible light. But luminescence can also refer to infrared and other forms of light the eye cannot see.
For a substance to become luminescent, the electrons in its atoms must absorb energy. The electrons get rid of this excess energy by giving off light. The energy to produce luminescence can come from various sources, including electric current, X rays, ultraviolet rays, and certain chemical reactions.
Light given off by fireflies and other living things is called bioluminescence. Luminescence that stops as soon as the energy source is removed is called fluorescence. Luminescence that lasts for seconds or even days after the energy source is removed is called phosphorescence. See Bioluminescence ; Fluorescence ; Phosphorescence .
