Objective

Have you ever wondered what makes certain things glow under black lights? In this experiment you will find out how to make things glow in the dark.

Introduction

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.

Required

• a black light
• petroleum jelly

Procedure

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.

Glowing Hands

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.