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[Text: QQ: How do blacklights make things glow?]

Hank: Whether it was at a club, or an amusement park, or a college dorm room, you have probably seen them before: Blacklights, with their eerie purple light that make your buddy’s teeth glow in a really unnerving way, also yours, by the way. And show off every little stain on your white t-shirt. Blacklights can be both fun and useful. But how do they work, and why do they only make certain things light up with that classic radioactive-looking glow?

Well, on a basic level, a blacklight is pretty much like any other light source like a spotlight, or even a candle flame: they all emit electromagnetic radiation. But, as you might have noticed, a blacklight doesn’t exactly brighten up a room like a normal lightbulb. That’s because blacklights are designed to mostly emit only higher frequency ultraviolet light, which is outside the visible spectrum, meaning human eyes can’t detect it.

Blacklights use special filters or coatings to absorb or block most of their light, allowing only UV-A and a little bit of visible light to pass through. You’ll see a purplish glow around the bulb, but most of the light coming out of the bulb is essentially invisible to us. So, under a blacklight, most things look dark to us. But some things glow brightly, because they contain compounds known as phosphors.

Phosphors are substances that absorb that electromagnetic energy coming from a light source, and then fluoresce, meaning they emit the light that they’ve absorbed at a lower, visible frequency. In other words, they glow. When you shine a blacklight on a phosphor, what you’re actually doing is getting the object to convert that invisible UV radiation into light that you can see.

Lots of stuff contains phosphors, highlighter ink, glow-in-the-dark toys, neon paints, tonic water, even a lot of laundry detergents, which is why white clothing glows so brightly under a blacklight. Human teeth, fingernails, and certain bodily fluids like blood, urine, and semen all contain natural phosphors, which is why forensic investigators might bust out a blacklight at a crime scene.

These lights can even be helpful in diagnosing certain topical fungal or bacterial infections. Beyond lending ambiance at the club and checking your hotel sheets for various... fluids, phosphor-detecting blacklights can be useful in other ways. For example, machinists looking for small, invisible leaks might inject a little phosphorescent dye into a fuel line and then whip out a blacklight to hone in on the problem. Security officers and bouncers often use blacklights to verify IDs, and some countries embed phosphor strips in their more valuable paper currency, which means you can use a blacklight to check for counterfeiting. And of course, when it comes to personal decorating, it’s the only thing in the world that could make that vintage neon velvet Led Zeppelin poster look even sweeter.

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