A variety of compounds emit a blue, green or yellow glow when exposed to ultraviolet light, also known as black light -- and many can be found in your cocktail glass. They do so by absorbing the energy from the ultraviolet radiation and then emitting it in a the visible spectrum. Which color they glow depends on the molecular structure of the compound. Don't just save these libations for Halloween; screw in your ultraviolet bulb, lower the lights and spice up any cocktail occasion with a cool liquid glow.
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Glow Blue With Quinine
Quinine, the compound that gives tonic water its bitter flavor, glows blue when exposed to black light. Thus, any drink made with tonic water will also glow in the dark, from a simple gin and tonic to a fancy party punch. Use transparent liquids as mixers or you'll dull the drink's luminosity. You can also freeze tonic water in an ice cube tray for glowing ice cubes.
Glow Green with Energy Drinks
Several brands of energy drinks contain large amounts of taurine, guanine and B-complex vitamins including thiamine, niacin, riboflavin and vitamin B12, all of which glow yellow under black light. Use energy drinks as a mixer with rum, vodka or whisky. You can also add yellow-hued mixers such as pineapple juice or lemonade to enhance the effect.
Technically, milk doesn't actually glow green under ultraviolet light; rather, it reflects the black light off its opaque white surface. Nonetheless, the effect is still striking in a dark room. Vanilla ice cream has the same property, by the way, so your virgin vanilla shake will also glow in the dark.
Bananas glow blue under black light -- the riper they are, the brighter they glow, so use the ripest bananas you can find if you're going to try them as a black-light drink ingredient. Pureed banana is essential to the classic banana daiquiri cocktail, and you'll find it in many other tropical-themed blended mixed drinks as well, from the banana colada to the Bananas Foster martini.