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Flying circus of physics

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Wintergreen glow in the closet

You and a friend first adapt your eyes to darkness for about 15 minutes in a closet or outside on a moonless night. Then have your friend chew a wintergreen Life Saver candy (a candy in the shape of a marine lifesaver and infused with the oil of wintergreen) with the mouth as open as possible so that you can see inside. Why does each bite initially produce a faint flash of blue light, and why do later bites fail to produce the light? (If you don’t want to eat the candy, squeeze it with a pair of pliers until it fractures.) Why does tonic water have a faint blue tint?

Whenever a bite breaks one of the mint’s sugar crystal into pieces, the pieces will probably end up with different charges. Suppose a crystal breaks into pieces A and B, with A negatively charged and B positively charged. Some of the electrons on A will then jump across the gap to reach B. Because air has moved into the gap after the crystal breaks, these electrons jump through air. Those that collide with nitrogen molecules in the air transfer energy to the molecules, exciting them. When the molecules de-excite, they emit in the ultraviolet range, which you cannot see. However, the wintergreen molecules on the surface of the candy pieces absorb ultraviolet light and then emit blue light, which you can see---it is the blue light coming from your friend’s mouth. This process of absorbing light in one wavelength range (here, in the ultraviolet) and then emitting light in another wavelength range (here, blue light) is called fluorescence. 

The quinine in tonic water is like wintergreen oil in that it absorbs ultraviolet light and then emits blue light, giving tonic water its faint blue tint. You can see the tint better if the tonic water is near a fluorescent bulb in an otherwise dark room. The quinine will then convert some of the ultraviolet light from the bulb into blue light.The effect is decreased if the illumination is through a plastic or glass wall such as with a bottle of tonic water, because plastic and glass absorb ultraviolet light. The effect is increased if you illuminate the tonic water with a black light (ultraviolet) lamp.

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