How Do Blacklights Make Things Glow

Whether it was in a club or an amusement park or a college dorm room, you have probably seen them before. Black lights, 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. Black lights 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?

black lights in a club

Well, on a basic level, a black light 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've noticed, a black light doesn't exactly brighten up a room like a normal light bulb. That's because black lights are designed to mostly emit only higher frequency ultraviolet light, which is outside the visible spectrum, meaning human eyes can't detect it. Black lights use special filters or coatings to absorb or block most of their light, allowing only UVA and a little bit of visible light to pass through. You'll see a purple-ish glow around the bulb, but most of the light coming out of the bulb is essentially invisible to us.

Under a black light, 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 black light 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 black light. 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 black light at a crime scene. These lights can even be helpful in diagnosing certain topical, fungal or bacterial infections.


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