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Sugar and Spice...and Light
| Article
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15206 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
4 / 1989 |
937 Words |
| Author
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Linda M. Sweeting
Linda M. Sweeting is professor of chemistry at Towson State
University in Towson, Maryland. She first viewed
triboluminescence while working in a dark laboratory and has
recently authored several papers on the spectra and structure
of triboluminescent crystals. |
Light emission by solids when they are fractured has been observed for millennia. The first written observation was probably that of Francis Bacon: In 1605 he reported the light emissions of cane sugar (sucrose) in The Advancement of Learning. The phenomenon is most commonly called triboluminescence, form the Greek verb meaning "to rub." It is very common in natural materials: minerals and rocks such as mica, quartz, and granite; plant extracts such as menthol and tartaric acid; and even bones. Modern man-made materials have added adhesives and plastics such as Plexiglas to the list. "Earthquake lights," light emissions before and during earthquakes, are probably triboluminescence in some cases. It has been estimated that at least one-third of all solids triboluminesce.
This phenomenon can be observed at home. Take into a dark closet or room several sugar cubes, some wintergreen candy, and some rolls of adhesive tape. Let your eyes adapt to the darkness for about five minutes. To see the triboluminescence of sugar, strike the cubes against each other as if you were striking a match, or use a mortar and pestle to grind it. To observe the triboluminescence of wintergreen candy, bite into it with your mouth open in front of a mirror, being careful not to get the candy too wet. Then fold a piece of tape with the adhesive sides together, leaving two tabs to pull the faces apart again, or just pull it off the roll, to witness the effect.
Triboluminescence is just one of many forms of luminescence, or light emission. Luminescence results when an atom or molecule receives energy that is sufficient to excite an electron into an unusually high energy state; when the electron energy drops back to normal, the excess energy is released from the molecule as light. The color or spectrum of the light depends on the structure of the atom or molecule; some of the light may be ultraviolet or infrared, and thus invisible.
The spectrum of the triboluminescence emission reveals a great deal about the cause of the phenomenon. Some solids, such as sugar, quartz, and Plexiglas, emit bluish light with exactly the same spectrum as lighting. This spectrum is produced by electrons colliding with nitrogen molecules during an electrical discharge through the air. It is thought that the strain produced in the crystal during the cracking causes charges to be separated briefly. If the crystal cannot conduct electricity, the electrons must jump through the air in the cracks to meet the positive charges, exciting the nitrogen en route. Most of the luminescence is ultraviolet, but we are able to see the small fraction in the blue region of the
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