Glowing, Glowing, Glowing, . . . . Gone?


And then there is glow that is not related to making something hot, called luminescence. The kind of luminescence depends on specific details of the quantum structure of the material, but falls into some major categories:

Fluorescence Luminescence in which visible light is emitted from a substance under excitation by ultraviolet light or other forms of electromagnetic radiation. The light is given off only while the stimulation continues. Fluorescence of certain rocks and other substances has been observed for hundreds of years. The wavelength of the fluorescent light is always greater than that of the exciting radiation.
Luminescence produced by certain substances after absorbing radiant energy or other types of energy. Phosphorescence continues even after the radiation causing it has ceased. Energy is absorbed by a phosphorescent substance, causing some of the electrons of the crystal to be displaced. These electrons become trapped in potential troughs from which they are eventually freed by temperature-related energy fluctuations within the crystal. As they fall back to their original energy levels, they release their excess energy in the form of light.
Chemiluminescence The name given to those cases in which chemical action produces light without any great rise of temperature. Phosphorus exposed to moist air in a dark room shines with a soft light due to slow oxidation. Decaying wood and other vegetable substances often exhibit the same property.
Thermoluminescence Some materials that do not emit light at ordinary temperatures in a dark room begin to do so if they are heated to a temperature below a visible red heat. In the case of chlorophane, a variety of fluor-spar, the heat of your hand is sufficient.
Electroluminescence Lluminescence due to electrical causes. Many gases are phosphorescent for a short time after an electric discharge has been passed through them, and some solid substances are strongly phosphorescent when exposed to a beam of electrons.
Triboluminescence Luminescence excited by friction, percussion, cleavage or other mechanical means. Popularly, Winter Green Lifesavers are famous for this effect. The ultraviolet light produced by cleaving sugar crystals is converted into a flash of visible light by the organic dyes and flavoring of the wintergreen mint.


Fluorescence - Cool Glow
Triboluminescence - WintOGreen Lifesaver

While all forms of luminescence are interesting to think about, our focus is on phosphorescence this time.


© 2004-2008 - 4Physics