A few summers ago I went spelunking in South Dakota. We had all the essential gear including three lights each, rope, and wintergreen Lifesavers. We turned on our headlamps and headed into the chilly darkness. We went past the remnants of an old campsite, through a long tunnel called the Sternum Buster (aptly named [it was so small the kid in front of me lost his pants, but that’s another hilarious story]) and finally we emerged into a chamber. We took some time to go down to look at the water table where the water was so pure it was hard to see, but then it was time for business. In a cave it is so dark that you literally can't see your hand. That’s why we turned off our headlamps and handed out the wintergreen Lifesavers. In the pitch blackness as people munched on the Lifesavers, flashes illuminated their mouths like a tiny fireworks show, and then it was completely dark again.
Why do Lifesavers flash when you chew them? Well, first of all, it can’t be just any Lifesaver, but it can be just any opaque candy. Transparent candy does not work, but sucrose and wintergreen both can emit light. This means that if you are in a dark place you can strike sugar cubes on each other and they will make blue or green light. In fact, the spectrum produced by doing this is exactly the same as in lightning. 1 In an electric storm clouds become charged. The bottom of a cloud with a large negative charge will create an electric field. When the field ionizes the air and finds a path to the positively charged earth, electrons explode down the completed circuit. When the rapid movement of the electrons excites nitrogen in the air, we see a blinding flash of lightning.2
So do lifesavers make lightning? Nope, but sugar produces light in a similar way. Lightning happens when a negative charge (a cloud) is separated from a positive charge (the ground). When a sugar crystal breaks, one side will likely have a net positive charge, and one side will have a negative charge. Electrons will rush across the gap, just like in lightning. When electrons pass through the air they excite nitrogen, which emits light at certain wavelengths. A man named Kirchhoff once determined laws about this emission based on his experiments. His second law says: "A hot, transparent gas produces an emission line spectrum- a series of bright spectral lines against a dark background."3 In other words, when electrons travel across the gap, they create heat, and when a gas is heated it emits a specific light.
The specific light that nitrogen gives off just happens to be just right to activate fluorescence in wintergreen. Fluorescence is a process in which energy is absorbed and released at a longer wavelength (lower energy). Wintergreen absorbs light at the energy level of nitrogen emission and releases it as lower energy blue light. The light given off by wintergreen just so happens to look brighter to us because it is more within the visible spectrum than the nitrogen light, which is mostly ultraviolet.4
So now we know how the process works. It is actually more complicated than just one interaction. First, nitrogen molecules are excited to higher energy levels by electrons bridging the gap between charges. When their energy falls back down they release light which excites wintergreen molecules to higher energy levels, which fall back down and produce the blue light we see in the first few bites. You have to hurry though, because water from your mouth will short out the gap; only the first few bites will flash.
This process is all made possible by fluorescence.To illustrate exactly how this process works, below is a Jablonski diagram.
5
The diagram shows three stages. Below is a table that describes what happens at each stage.
| Stage | Process |
|---|---|
| Stage 1 | The compound is excited by photons from an outside source |
| Stage 2 | Energy is lost to surrounding environment |
| Stage 3 | The compound releases a photon of lower energy |
See? Simple! So next time you're lost in the dark without a flashlight, just try to break something. Maybe if the pieces have different charges you will get a few flashes.
Sources
1 Sweeting, Linda M. “Scientific Experiments at Home: Wintergreen Candy and other Triboluminescent Materials.” Towson University. http://pages.towson.edu/ladon/wg/candywww.htm. Accessed 10/19/09.A site about experiments with triboluminescent candy! This was a useful, in-depth look at why Lifesavers flash. This is definitely a credible source, this professor knows what she is doing.
2 “How Lightning Works.” How Stuff Works. http://science.howstuffworks.com/lightning4.htm. Accessed 10/19/09.A basic explanation of how Lightning works. This source is useful to help us understand the first part of this process. This source is probably credible, although the website is run by Discovery. The corporation could have bias, but it is safe to assume their information is generally correct.
3 Freedman, Rodger A. and William J. Kaufmann III. Universe, Eighth Edition. New York, 2008.An explanation of the physics of the Universe. This source usefully explains the processes behind light emission. This is definitely a credible source, it is a textbook.
4 See Sweeting, Linda M.
5 "Introduction to Fluorescence Techniques" Invitrogen Corporation, 2009. Accessed Oct 26, 2009. http://www.invitrogen.com/site/us/en/home/References/Molecular-Probes-The-Handbook/Introduction-to-Fluorescence-Techniques.html.An explanation of the fluorescence process. This site explained the Jablonski diagram. Again, this is a corporation, but their site is incredibly in-depth and the credibility seems to be very high.
By Nicholas Barkdull
