IF YOU STAND IN FRONT OF a mirror in a dark room long enough for your eyes to adapt to the darkness and then start chewing on a piece of hard candy, you may see in the reflection of your open mouth a faint light like the glow from the dial of a luminescent watch. The glow from the candy is triboluminescence (from the Greek tribein, to rub) and can be demonstrated with most hard candies that are made with granulated sugar. It is the sugar, or more precisely the molecules of gaseous nitrogen around it, that accounts for the glow.

I first noticed the phenomenon when I was standing in front of a mirror in a dark room while chewing on a wintergreen Lifesaver. As I crunched the confection I could see in the reflection that the inside of my mouth was aglow with a blue green light. The phenomenon has been known since at least the 1 7th century, when the Accadèmia del Cimento in Florence published a description that in a contemporary translation reads: "Of Bodies affording Light.... Besides Firestones there are other Conservatories of Light; for by striking them together or breaking them in the Dark, they Sparkle. Such are White Sugar, Loaf Sugar and Sal Gemme [rock salt] in the Stone; all which being broken in a Mortar, give forth so great a Light as distinctly to discern the sides of the Mortar and the shape of the Pestle thereby." At about the same time Robert Boyle noted that "hard sugar being nimbly scraped with a knife would afford a sparkling light." Many investigators since then have explored the phenomenon, but the nature of triboluminescence was not understood until this century. Even now the details have not been fully worked out.

Until quite recently the study of triboluminescence focused on determining what substances glow, examining the spectrum of the light and exploring how the glow might be correlated with the physical properties of the substance. Much of the modern research has been done by Jeffrey I. Zink and his associates at the University of California at Los Angeles. They and others have ascertained the source of the light in sugar and certain other substances. An explanation of the relation between the glow and the pressure on the substance has remained elusive.

A good many materials display triboluminescence, not always in the same form as ordinary sugar (sucrose) does. Zink and others have found that the sugars D-glucose, lactose, maltose and L-rhamnose emit the glow but the sugars fructose, cellobiose, fucose, galactose and mannose do not.

Early in this century the spectrum of the glow from sucrose was shown to be the same as the spectrum of gaseous

nitrogen. It was clear that the glow did not originate in the molecules of the sugar itself, because they would yield an entirely different spectrum. The spectrograph shows that the triboluminescence of sugar has several bright peaks near the ultraviolet region of the spectrum. Strong emissions appear at 330 and 360 nanometers, weaker ones near 315 and 380. The visible glow is fainter, lying between 380 and 435 nanometers (the violet and blue regions of the spectrum). If the eye could see farther into the ultraviolet, the glow would be brighter because the strong emission peaks would then be visible.

The crushing of sugar crystals somehow excites the molecular nitrogen associated with the sugar. An analysis of the spectrum indicates that the nitrogen is excited from its lowest molecular energy state to the ³¹_u state. It quickly drops to the ³¹_g state, emitting light in a certain assortment of wavelengths. It is these wavelengths that can be identified as the assortment unique to molecular nitrogen.

The role of nitrogen can also be demonstrated by removing from the sugar a much nitrogen as possible. When sugar is crushed in a vacuum, the glow is dimmer. It is still there because even in a vacuum some molecules of nitrogen are adsorbed on or absorbed by the sugar. When nitrogen is entirely eliminated, as Zink has managed to do, the glow disappears.

The importance of the ambient gas is similarly revealed when other gases are substituted for nitrogen. When the sugar is placed in an atmosphere of neon at low pressure, the glow from crushing the sugar is red. The neon is somehow excited by the pressure; it then deexcites much as it does in neon signs. Neon's emission is strong in the red region of the spectrum.

Certain hard candies display additional colors. Zink and his associates have found, for example, that wintergreen Lifesavers give off more of a blue green color than other flavors, which have the weak blue emission of sucrose. The reason is that the ultraviolet radiation of the nitrogen is absorbed by the methyl salicylate (oil of wintergreen) with which the candy is flavored. When the methyl salicylate deexcites, it emits in a broad range from about 400 nanometers to about 540. The chewer sees a composite of the spectrum from molecular nitrogen and methyl salicylate.

I demonstrated triboluminescence by grinding table sugar and various sugar products in a beaker after giving my eyes time to adapt to darkness. What you grind with does not seem to matter. I had good results with either the blunt end of a metal table knife or a wood dowel. Nor does it matter whether the beaker is made of glass, plastic or metal. (Be careful with glass. Since you are working in the dark, you could break a glass beaker by applying pressure in the wrong way.)

The amount of light emitted by sugar seemed to diminish as I continued grinding. Sugar made slightly damp by the humidity in the room gave off a fainter glow. Sugar dissolved in water did not glow unless I hit an undissolved crystal with the grinding tool.

Many of the liquids and powders for washing clothes have a component that absorbs ultraviolet and emits in the visible spectrum. (The aim is to make the clothes look whiter.) I added one of these products to sugar but saw no difference in the glow.

I also studied the glow from several flavors of Lifesavers. To spare my teeth I followed Zink's suggestion of crushing the candy with a pair of pliers. Wintergreen yielded the most easily seen light, apparently because of the emission from the methyl salicylate.

To study the spectrum I taped a replica diffraction grating to my eyeglasses. If I looked directly at the glow, I received the central peak of the pattern developed by the grating. This peak overlaps all the colors from a light source, however, and prevents a determination of the spectrum. The first side peak from a diffraction grating separates the colors from a source, but could not make out the side peak, apparently because the light from the candy was so faint. Even a brighter side pea might not be discerned because the light is so faint that the retina would have difficulty distinguishing the colors.

You can buy an inexpensive replica diffraction grating from the Edmund Scientific Company (101 East Gloucester Pike, Barrington, N.J. 08007). If you have made any of the spectrum analyzers I have described in other articles for this department, you will be able to separate the spectrum of a piece of hard candy. A procedure for doing such an experiment is given in the article by Rebecca Angelos, Zink and Gordon E. Hardy cited in this month's bibliography.

I found triboluminescence in tablets of saccharin and crystals of tartaric acid and rock salt. Table salt and all soft candies failed to glow. Although soft candy is likely to contain sugar, the crystals may be too small or too difficult to stress (because of the plasticity of the candy) for a glow to result.

Sometimes hard candy will glow as a result of a sudden shock from a source other than crushing. If a Lifesaver is dropped into liquid nitrogen, the sudden drop in temperature creates enough stress to generate a brief glow. (If you want to repeat the experiment, be extremely careful with the liquid nitrogen. It can be dangerous if it is handled improperly.)

Many other substances display a triboluminescence that originates in the material itself rather than in the ambient gas. You can recognize such an origin if the spectrum of the triboluminescence matches the spectrum emitted when the substance is heated-or illuminated. Here again the connection between the applied pressure and the excitation of the emitting molecules is not well understood. It could be the result of chemical reactions. It could also be thermal, arising because the pressure distorts the molecules, alters the interactions of adjacent molecules or moves dislocations in crystals of the substance.

The glow from hard candy seems to be due to electric discharge rather than to chemical or thermal mechanisms. The discharge results from a separation of charges brought about by the stress on the material. Even though the material is usually not strongly charged, stress liberates ions and semifree electrons. The glow may then result from the recombination of positive and negative ions. Or it could result when the semifree electrons combine with positive sites in the material, giving rise to light. A third possibility is that the electrons are accelerated by electric fields between the charged sites and collide with molecules, exciting them so that they emit light when they drop to lower energy levels.

The glow from sugar appears to be attributable to an electrical action that results in molecular excitation by collision. When the sugar is pressed, electrons are freed at various points within it. The electrons jump across a crystal of the material or between crystals to reach positively charged sites. In the process they collide with molecular nitrogen that has been adsorbed on or absorbed by the sugar. The electrons also collide with molecules of nitrogen gas. Whatever the type of collision, the electrons transfer some of their kinetic energy to the molecular nitrogen, raising it to an excited state.

In sugar the creation of charged surfaces appears to be caused by the fracture of the sugar crystals. A fracture leaves charged surfaces that are almost immediately coated with molecular nitrogen from the air. The stage is set for moving electrons to make the nitrogen glow.

Zink and his associates have investigated the rate at which cracks propagate through crystals of sugar. They find that a single crack and its associated glow should persist for about a microsecond. Chewing on a Lifesaver or crushing it with pliers generates a longer glow because of a progression of cracks. The brightness of the glow depends on the size of the crystals. I believe the glow diminishes as I continue crushing a piece of hard candy because the crystals become smaller.

Another material that displays triboluminescence is tape coated on one side with an adhesive. E. Newton Harvey of Princeton University demonstrated in 1939 that several kinds of tape glow in the dark when a strip is pulled rapidly from the roll or off certain surfaces such as glass. The glow can also be seen if two layers of tape are stuck together and then pulled apart. Occasionally it appears if you run your finger along the uncoated side of a length of tape.

As with sugar the glow results from a discharge through the ambient nitrogen gas. The spectrum matches that of molecular nitrogen. If a tape is unrolled in an atmosphere of neon at low pressure, the glow is red instead of blue.

Apparently the manipulation of the tape builds up small areas of charge. When these areas are separated, the electrons in the negative areas leap through the gas to reach the positive areas, exciting the ambient nitrogen on the way. Similar glows can be seen in mica as it is split and on rare occasions in a stretched rubber band as the stretch is released suddenly.

Harvey described how to demonstrate that pulling a piece of tape from a substrate results in a separation of charge. He stuck a length of tape on a sheet of glass and pulled it off. If he then held the tape near the glass, it was attracted to the glass by the charged sites that had developed on the two surfaces. Harvey ascertained the sign of the charge on each surface by means of charged pith balls. A positively charged pith ball was attracted to the tape, indicating that the tape had been made negative by its removal from the glass. Similarly, a negatively charged pith ball was attracted to the glass. This is not to say that the tape is necessarily all negative and the glass all positive, merely that localized pockets of charge on each surface are dominated by charge of one sign.

Conventional optical devices such as lenses and prisms work by refracting light, that is, by redirecting it at their surfaces. Light is refracted when it crosses a boundary between two materials in which its effective speeds differ One way of keeping track of the degree 0f refraction is to assign to each material an index of refraction; the slower light moves in the material, the higher the index of refraction is. For example, glass has an index of about 1.5, air an index of slightly more than 1.

Light is refracted on entering and leaving a glass lens but is not refracted inside the lens, since the index of refraction does not change within the glass. If the sides of the glass were parallel and flat, the refraction of light at the surface where the light enters the glass would be canceled by the refraction at the surface. where it emerges. A lens has curved surfaces in order to create a net change in the direction in which the light propagates. A convex lens redirects parallel rays of light so that they cross at a focal point after passing through the lens.

A gradient-index lens (called, for obvious reasons, a GRIN lens) is unconventional in that its index of refraction varies internally. In one type of GRIN lens the index varies along an axis perpendicular to the optical axis (the axis along which the light is directed). The center of such a lens might have an index of refraction higher than that of the periphery. When light passes through the lens, the rays near the optical axis are deflected weakly but those away from the optical axis are directed strongly toward it. Then even if the lens has flat, parallel exterior surfaces, it can focus light like a conventional convex lens. If the surfaces are curved, the light is refracted even more.

A second type of GRIN lens has its gradient along the optical axis rather than perpendicular to it. The index at the front surface is different from the one at the rear surface, and it varies smoothly between them. A third type of lens would have a spherical gradient in its index of refraction. For example, the index might decrease outward from the center of the lens in a spherically symmetrical way. Although I do not know if any lens of this third type has ever been made, James Clerk Maxwell studied its - optics more than a century ago. A prototype is the crystalline lens of the eye.

Several simple applications have been found for GRIN lenses, but the lenses are not common because it is hard to make them. Jurgen R. Meyer-Arendt of Pacific University and his student Mark R. Zilm recently described to me an easy way of making a simple GRIN lens. They have made one that behaves like a concave cylindrical lens.

In essence their lens consists of clear epoxy into which potassium nitrate has been allowed to diffuse, thereby creating the gradient in the index of refraction. A container for the material is made with two square pieces of glass, each piece 50 millimeters on an edge and 2.05 millimeters thick. They are held parallel and separate by thin pieces of wood (matchsticks) that are 50 millimeters long and three millimeters square in cross section. The wood was originally balsa, but Meyer-Arendt and Zilm later found that the hard wood of matchsticks is better. The wood is glued along three edges of a piece of the glass with a silicone seal glue. (Other types of glue might not work as well because the glue might diffuse into the epoxy.)

The fourth side of the container is left open so that the epoxy and potassium nitrate can be poured in. Potassium nitrate

(KNO₃), otherwise known as saltpeter, can be bought in a drugstore. The epoxy chosen by Meyer-Arendt and Zilm is Envirotex polymer coating, which is sold in stores that deal in materials for paper-cutout decoration (decoupage). The potassium nitrate is packed to a uniform depth of about two millimeters at the bottom of the container, which is then filled with the epoxy. Meyer-Arendt and Zilm modified the usual procedure for mixing epoxy because they wanted the material to take longer to harden so that some of the potassium nitrate would have time to diffuse into it. The standard ratio of hardener to epoxy is 1:1; they made it 1:4. They tried even smaller amounts of hardener but found that air bubbles were trapped in the mixture when it hardened.

After three weeks in which the container was left undisturbed the potassium nitrate had diffused upward into the epoxy and the entire mixture had solidified. The result was a GRIN lens with an index of refraction that varied upward. The index was highest at the bottom, where the potassium nitrate was concentrated, and lowest near the top, where no potassium nitrate had penetrated.

Meyer-Arendt and Zilm devised a simple experiment to demonstrate the refractive characteristics of their device. First they etched a grid on one of the pieces of glass at the side of the assembly. Each rectangle in the grid was three millimeters high and four millimeters wide. The GRIN lens was then set on one edge so that what had been the base was now vertical.

The next step was to direct a laser beam through one section of the grid. The light passed through the lens and fell on a screen 10 meters away. Ascertaining first where the laser beam hit the screen when the lens was not in place and then where the beam hit after passing through the lens, Meyer-Arendt and Zilm could measure how much the lens deflected it. They repeated the measurement along a line of grid sections ranging from the high-index part of the lens to the low-index part.

The refraction of a laser beam passing through the GRIN lens near what was originally the bottom is shown in the top illustration above. The beam travels through a gradient in the index of refraction. The part of the beam passing through the area with a higher index is delayed with respect to the part that goes through the area with a smaller index. As a result the beam is deflected toward the side that was originally the base.

The optical properties of this GRIN lens are something like those of half of a concave cylindrical lens, as is shown in the illustration on the left. Near the areas with a high gradient in the concentration of potassium nitrate light is strongly deflected, as it would be in the outer part of the concave lens. The deflection in the areas with a smaller gradient of potassium nitrate is much as it would be in the center of the concave lens. If the diffusion of the potassium nitrate into the epoxy could be precisely controlled, one could presumably make a GRIN lens to match any conventional lens.

If you make a GRIN lens the way Meyer-Arendt and Zilm did, you might want to investigate its chromatic properties.

Unless you have access to laser light of various colors, you will have to work with partially coherent light from a pinhole. The source could be any bright light filtered to narrow the range of color in the beam. You will need a mask or a stop to confine the light to only a small area of the GRIN lens.

A good demonstration of gradient index refraction published by William M. Strouse is cited in the bibliography [below]. He directed a beam of light through a pair of lenses (to narrow it) and then into a tank filled with a solution of sugar in water. The tank was two centimeters wide, eight high and 20 long. Strouse put in three centimeters of water and added three lumps of sugar, which he allowed to dissolve undisturbed. As the sugar diffused through the water it created a gradient that was highest near the bottom and lowest near the top. The index of refraction therefore changed from high to low in the same direction. When the beam was directed into the solution near the bottom of the tank, it curved downward and was reflected off the bottom. The reflected beam continued to curve. With a long tank the beam would bounce along the bottom.

Seen from above, the beam maintains the same width as it crosses the tank. Seen from the side, it narrows periodically. This partial focusing is also due to the gradient of the index of refraction. The gradient redirects the rays in differing degrees, so that the beam narrows in some parts of the solution and widens in others. Strouse said the phenomenon is enhanced if the lenses installed to narrow the initial beam are removed. The beam gets wider and is therefore easier to follow.

Bibliography

BOUNCING LIGHT BEAM. William M. Strouse in American Journal of Physics, Vol. 40, No. 6, pages 913-914; June, 1972.

TRIBOLUMINESCENCE SPECTROSCOPY OF COMMON CANDIES. Rebecca Angelos, Jeffrey I. Zink and Gordon E. Hardy in Journal of Chemical Education, Vol. 56, No. 6, pages 413-414; June, 1979.

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