Turn on a black light in a dark room, and you'll see a dim, purplish glow. But, the bulb also produces ultraviolet light, which you can't see.
As you know, -- white T-shirts, fluorescent posters, toothpaste, and petroleum jelly, just to name a few. Why? Phosphors in these objects emit visible light in response to UV radiation from the black light. (Interestingly, the phosphors in white T-shirts come from the detergents we use to wash them.)
Here's an oldie-but-goodie phone hack that's fun for Halloween, takes only a few minutes, and probably won't cost you a dime. We're going to turn a smartphone's camera flash into a black light!
OK, so maybe it's not the world's greatest black light -- call it Black Light Lite -- but it does work. Here's how to do it.
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Cut off a small piece of tape and cover the flash.
Color the first layer of tape -- just the part covering the flash --with blue Sharpie ink.
Apply a second layer of tape. Use the purple Sharpie to draw over the blue-inked area.
Turn out the lights (if it's nighttime) or find a windowless room and shut the door. Turn on the phone's flashlight and enjoy the eerie, bluish-purple glow.
Let the glowing begin! Here's an example of neon-colored highlighter ink -- a yellow Sharpie, in fact -- glowing under my iPhone 6's black light.
White paper glows too, so I created a paper airplane for the occasion.
Halloween supplies are often black-light friendly.
The Skull of Doom glowed as well.
What else glowed? A mouthful of toothpaste, but that photo (of me) was so disturbing that I chose not to upload it here.
Again, don't expect the vibrant glow you'd get from a dedicated black light, but this simple hack is easy and fun, particularly with kids.