How Black Light Works

What would Tron be without black light? Or television crime scene investigations, for that matter? Just screw in a special lightbulb, and suddenly everything glows in an awesome way. But how does this actually work?

Find out the secrets of ultraviolet light, why some biological substances glow, and how to make glowing food.

Black light seems like a pretty futuristic invention — but in fact, it's just regular, sterile fluorescent lighting with a bit taken out of the bulb. Regular fluorescent bulbs are tubes filled with a noble gas and some mercury gas mixed in. When energy is pumped into the mercury atoms via electricity, they emit photons, but not photons that humans can see. Mercury emits ultraviolet light, which is both unhealthy and invisible, since it's much more energetic than visible light.

The reason that most fluorescent lighting is visible, while black light is not, is the hazy white on the outside of the fluorescent bulb. The bulb is coated with phosphors. These phosphors, when hit with photons, create heat as well as light. As a result of that particular combination of heat an light, the photons that the phosphors emit are just a little less active than the photons that the phosphors absorb. The loss of energy kicks the photon into the visible light range, letting humans see it.

Black light bulbs have phosphor coating that absorbs the higher-end UV light, but lets the relatively harmless and low-energy UVA waves get through. Those UVA waves zoom around the room and hit other phosphors; ones that we usually don't notice. It's the phosphors liberally sprinkled around our daily lives that make black light so neat.

First let's talk about the phosphorescent things that are sprinkled a little close to home. Many bodily fluids, including spit, urine, and semen, glow under black light. After calcium, phosphorous is the most abundant mineral in the body. It helps the kidneys filter water, which is one of the reasons that urine glows so brightly. There are also a whole group of enzyme called phosphotases, which remove phosphorous from a substrate. It is these phosphotase enzymes that glow in fluids like semen. A major reason we don't constantly see a glow from the world around us is the brighter visible light drowns it out.

How Black Light Works


Which isn't to say that we don't perceive the phosphorescence. Paper is treated with phosphorous, as are white clothes and laundry detergents. These added phosphorous compounds help clothes and paper have a bright, marketable white glow.

What can you do to make a black light party pop? Luckily, there's hidden phosphorous all around us, and not just in your spit and semen. Bananas are extremely cool in black light, especially around their spots, which have deposits of phosphors. Banana spots glow in leopard-like patterns under black light. Chlorophyll glows red, and vaseline glows blue. Tonic water glows in black light. Vitamin A and the B vitamins thiamine, niacin, and riboflavin will all glow under black light, especially if they're soaked in vinegar. Simply grind up vitamin tablets, soak them in a little vinegar, and either leave them around or serve them as salad dressing. Just be sure to explain to your friends that they're vitamins. Otherwise, they might believe you're trying to serve them your body fluids.

Top Image: Tron Legacy via LA Times

Second Image: NSF

Via UMM, How Stuff Works, About.com, and Slate.