Have you ever wondered what makes certain things glow under black lights?

For this experiment you will need:

• a black light
• petroleum jelly
• a piece of paper

First we’ll use the 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. Use more jelly if you need to – but this probably isn’t the time to write a long speech! 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?

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 a spectrum of colors ranging from red, through orange, yellow, green, and blue, to violet or purple. Beyond violet light in the spectrum is ultraviolet light, which our eyes cannot detect.

You may have heard of ultraviolet light if you know about sunburn. Sunburn is caused by a type of ultraviolet light, which scientists call “ultraviolet B” (UV-B). UV-B is higher in energy than the light from black lights, which is called “ultraviolet A” (UV-A). Black lights will not give you a sunburn.

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 and emits it as visible light. So the phosphors in the jelly are absorbing the invisible ultraviolet radiation from the black light and emitting visible light.

Can you find anything else in your home that glows under black light?

One thing that usually glows brightly under black lights is a white shirt. Most laundry detergents contain “bluing agents” that are advertised as making the whites “whiter.” In fact, these agents are phosphors that respond to the UV-A radiation in normal light. The black light emphasizes their presence.

Another example of phosphors can be found on new $20 bills. As part of the government’s program to make currency harder to counterfeit, $20 bills issued since October, 2003, have a “security thread” that glows under ultraviolet light. The security thread is being introduced into $50 and $100 bills as well.

Glowing Hands

Can you think of a way to make your hands glow in the dark?

For this experiment you will need:

• a black light
• petroleum jelly
• latex gloves if you don't want to get your hands messy (caution: some people are allergic to latex gloves!)
• someone to turn on the black light for you.

If you have Latex gloves, put them on your hands. 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? Can you think of a way you could use this trick when telling ghost stories at night?