Kids, what makes certain things glow under black lights?  First, let’s talk about the light. The reason black lights are called "black lights" is because they give off very little light that our eyes can see. Visible light contains the colors of the rainbow:  red, orange, yellow, green, and blue, to violet or purple. Beyond violet light in the spectrum is ultraviolet light, which our eyes cannot detect.  You can buy a black light for about $5-$10 in a novelty store or some large home repair stores, and probably other places around halloween.  These are the lights that look a dim purple when lit up, but cause some things around you to fluoresce, or light up with bright colors.  These days they come as incandescent bulbs, fluorescent tubes, and cold fluorescent lights.  The incandescent bulbs get very hot to the touch, so be careful around them.

Please note:  All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.

First we’ll use petroleum jelly as a kind of invisible ink. Dip your finger into the jelly, then use your finger to write a message on the piece of paper.  When you’re finished, wipe any remaining jelly off your finger. Have the black light ready, then turn off the room lights and turn on the black light. Can you see the message? Why is something that you couldn’t see in room light now visible when you can’t see any light?

Can you think of a way to make your hands glow in the dark? If you have thin plastic gloves, put them on your hands (this is optional). Reach into the jar of petroleum jelly and scoop out enough jelly to cover both hands. Rub the jelly well over both hands, and then ask someone to turn off the lights in the room, and to turn on the black light. Hold your hand under the black light.  What do you see? If you prefer something less messy, just draw a picture on your hand with Vaseline, like a smiley face.  Turn on the black light in a darkened room to see your artwork.

If we can't see ultraviolet light, why does the petroleum jelly glow under the black light?  Most of the time when we look at an object, we see light reflected from the surface of the object. But with a black light, there isn't much visible light, so simple reflection of light doesn't account for how bright the jelly glows. Petroleum jelly contains substances called phosphors. A phosphor absorbs radiation, often in the ultraviolet, and emits it as visible light, in a process called fluorescence. So the phosphors in the jelly are absorbing the invisible ultraviolet radiation from the black light and emitting visible light.

Vaseline® Petroleum Jelly is a mixture of mineral oils, paraffin and microcrystalline waxes that, when blended together, create a smooth jelly that has a melting point just above body temperature.

With a little investigation, you might find many other things around your house that light up under black light.  Check out colored plastic parts, highlighter pens, and even powdered laundry detergent.  The detergent often has a whitener or optical brightener in it to make the clothes look whiter in daylight, and to replace optical brighteners added to the fabric while the piece is manufacturered.

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Kathleen Carrado Gregar, PhD, Argonne National Labs 
[email protected]
May 2013

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References:  http://scifun.chem.wisc.edu/homeexpts/vaseline.htm