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Fabric brighteners can liven up an old, faded shirt. But despite being in most detergents, fabric brighteners don’t actually clean your clothes.

And while they might make for a fun game of laser tag, they’re often banned in the military. All because of fluorescence: Fabric brighteners make your clothes glow. Clothing looks colorful because dye molecules reflect visible colors of light into your eyes.

And clothing fades when the dye in the fabric escapes into the water in your washing machine, or when ultraviolet light from the Sun breaks dye molecules apart. More washes, or more UV light, usually mean more faded colors. Besides not cleaning your clothes, fabric brighteners also don’t really stop either of those processes.

Instead, they act like a sort of dye-enhancer, helping whatever dye remains look its best. Fabric brighteners are part of a group of chemicals known as optical brighteners, which are members of an even bigger group of molecules that fluoresce. Fluorescent chemicals absorb light with some amount of energy and then emit light with less energy.

That can mean absorbing blue and emitting red, or absorbing red and emitting infrared. Or, for optical brighteners, it means absorbing ultraviolet and emitting blue. So when you go outside in clothes treated with an optical brightener, some invisible UV light might miss a dye molecule and hit a fluorescent molecule, which emits it as blue visible light.

But optical brighteners don’t just turn everything blue in sunlight. There’s not enough added to detergents to overpower whites or any colored dyes in clothes. This tiny bit of blue adds a little extra light coming from your clothes, and it helps drown out yellowing areas or small stains that can happen to white clothes over time.

So it makes whites look cleaner and creates more contrast with other colors. Long before optical brighteners, people were adding tiny amounts of blue dye to their laundry to achieve the same yellow-masking effects. So this isn’t a new trick.

And when there’s lots of visible light around, the fluorescence from optical brighteners doesn’t stand out very much. So clothes treated with them don’t tend to look like they’re glowing. But in places with lots of UV light and little else, there’s not much visible light to reflect off your clothes.

So the fluorescence starts to dominate, especially where there aren’t other dyes. That’s why white clothes tend to glow when you’re playing laser tag under all those UV-emitting black lights. Optical brighteners aren’t just a surprise at birthday parties, though.

The fluorescence also shows up through night-vision goggles, which can pick up on very tiny amounts of visible light. So detergents with optical brighteners tend to be banned in the military, where they could put soldiers in extra danger. So the next time someone tells you you’re glowing, it might just be your clothes.

But it’s probably you. Because, let’s be honest, you’re pretty great. Especially if you’re one of our channel members!

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