From toys to posters to t-shirts, items that glow in the dark have instant popularity among young and old alike. This mesmerizing effect is caused by a simple chemical reaction referred to as photoluminescence. The most common way of making an object glow in the dark is by incorporating a phosphor, a chemical that gives off visible light. When a phosphor is exposed to light, the light energy excites the electrons. As electrons absorb and then lose this energy, they too release it as light. Some common places phosphors are found include television screens, computer monitors and fluorescent light bulbs. So you’re staring at a group of densely packed, electrically charged phosphors right now!

In order to give off light, a phosphor must regularly recharge by being exposed to an energy source, just like a rechargeable battery after it has been depleted. If the phosphor is not exposed to an energy source, the glow will gradually fade until the electrons return to their original energy level.

There are numerous phosphors, but zinc sulfide and strontium aluminate (which glows much longer) are the two most common ones found in glow-in-the-dark items. These phosphors are mixed right in with the substance that is to glow, such as the plastic that will be molded into a glow-in-the-dark bowling ball or the screen for a glow-in-the-dark t-shirt.

Phosphors are defined by three separate characteristics: the type of energy required to energize their electrons, the color of visible light they produce and their persistence, or how long they will produce light after being energized. In a TV screen, an electron beam hits thousands of tiny elements with phosphors which glow either red, green or blue. In a fluorescent bulb, ultraviolet light energizes a mixture of phosphors that emit a light that appears white to the human eye.

Many materials are photoluminescent, but not all glow for a long time. The fluorescent bulb, for example, will glow as long as it is being energized, but stops immediately upon removal of the energizing light. Phosphorescent materials continue to glow after their energizing light is removed.

Items such as glow sticks produce light in a slightly different way than photoluminescent materials. The glow stick glows due to a chemical reaction called chemoluminescence, which means light energy is produced from a chemical reaction. External light or electricity is not needed. A glow stick contains two chemicals and a fluorescent dye. The chemicals are separated by a glass tube, which when broken begins a chemical reaction that can last from 6-12 hours.

Many years ago, radium paint was used to make watch and clock dials that glowed. As its name suggests, this element is radioactive. However, because it was used in such small amounts and was usually enclosed in a case, the radium paint did not present health risks. The use of radium is no longer allowed, but new watches may use a similar but less dangerous substance.