what you'll need
Also be sure to ask your mom, dad or another adult to help- they'll want to watch anyway when you tell them how cool this experiment is!
Does the candy glow in the dark before you crush it? Did you observe a flash of light as you crunched the candy with your pliers? It's not very bright, and it happens very quickly, but you should see a cool flash of light, like a spark. If it doesn't work at first, try again, and make sure the room is dark and your eyes are adjusted. What color is the light? Does it last very long? Does the candy get hot? Cold? If you have different types of candy or some sugar cubes, do you observe any differences in the light from each? If you have a sugar-free mint, does it make a flash of light too? If you have any other candies, try those and make observations.
This is not a magic trick or an optical illusion- you really did produce light from an ordinary piece of candy!. This is a demonstration of tribo-luminescence, which means to make light by rubbing, scratching or crushing something, which is very different from most of the ways you are familiar with to make light. Nothing is burning, glowing or getting hot like the filament in a light bulb (although there may be a little heat from the friction as the candy is crushed). It's also not a chemical reaction like the light-sticks you may have played with before. And though it does look sort of like the sparks you see when some metals (like iron) are scratched or scraped by very hard materials (like flint), those are actually produced by spontaneous combustion (burning) of the metal as freshly cut surfaces of very small pieces react with oxygen from the air.
Triboluminescence is not very well understood by scientists, but most think it is similar to the lightning you see during storms or during one of our cool electricity demonstrations, which is actually a type of plasma discharge. Many materials, like the sugar in this experiment, have a very special crystalline structure that causes electrons to be ripped away from the nucleus of their atoms. Since electrons have a negative electrical charge and the protons in the nucleus have a positive charge, separating these charges can create an electrical field strong enough to rip electrons off gas molecules in the air, a process called ionization, and creates a localized plasma. These plasma ions can then slam into other molecules and transfer energy which causes them to emit photons of light. Those are the flashes you observed. Most of the light energy is emitted in the ultraviolet (UV) part of the electromagnetic spectrum, which is just outside the range of light that is visible to humans, but fortunately there is a bit of violet and blue light that we can see.
You should have noticed that the flash of light from the Wint-O-Green Life Saver was significantly brighter, and may have also seemed to last a little longer. This is because wintergreen, the flavoring used in these mints, is a natural fluorescent dye called methyl salicylate. Fluorescent molecules can absorb energy of short wavelengths and emit their own light at a longer wavelength. In our experiment the wintergreen in the mint absorbs a lot of UV energy that we can't see, and emits greenish-blue light which we can see, so it appears much brighter to us.
variations and related activities
Try this experiment with ordinary table sugar (which you will have plenty of after you crush all your sugar cubes). Put a few teaspoons into a clear glass cup or dish (a Pyrex measuring cup works great) and slowly grind the sugar with a metal spoon to observe flashes of blue light. Try powdered sugar. You can also do the experiment in your mouth (bite very carefully so you don't break a tooth!) Be sure to look in a mirror so you can see the cool flashes of light in your mouth.
Here's another way to demonstrate triboluminescence: place a piece of Scotch tape onto a glass mirror or plate, then quickly rip it off in the dark. If you're lucky you should see a flash of light, but in our experience this experiment can be very finicky! Others have done this with Duct tape too.
As was mentioned earlier, triboluminescence is not fully understood. Maybe you will be the scientist to finally explain this phenomenon! Most scientists believe air molecules play a role and that light should not be produced if the sugar is wet or under water, but we have observed flashes of light in both cases. What about other liquids? Humidity in the air can also affect the results (this is especially true for the scotch tape experiment). Let us know what you observe in your experiments in the Comments blog below.
references and links to more information
Some photos and videos of triboluminescence light flashes:
Others instructions for this experiment:
Youtube video showing flashes of light from Duct tape:
X-rays from Scotch tape:
Wikipedia article on triboluminescence:
Journal paper from Towson State University researchers (yes, real scientists study this!):
Luz de Dulce - Triboluminiscencia (en español)
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