Glowing in the dark — also known as luminescence — simply requires chemicals that store energy when exposed to light. These special substances are called phosphors. This type of glowing is sometimes called phosphorescence.
Phosphors radiate visible light after being energized. This means you have to expose the items to light for a while before they will glow in the dark.
Phosphors then slowly release their stored energy over time. As they release the energy, they emit small amounts of light, which we see as an object glowing.
Sometimes glow-in-the-dark objects will only glow very weakly for a short time. Often, you have to place them in a very dark place to see their faint green glow. Newer glow-in-the-dark items may glow more brightly for several hours.
Over the years, chemists have created thousands of chemical compounds that act as phosphors. For glow-in-the-dark toys, manufacturers look for phosphors that can be energized by normal light and that glow as long as possible.
To make glow-in-the-dark toys, manufacturers mix their chosen phosphor into plastic and then mold it to the desired shape. Two of the most common phosphors found in glow-in-the-dark toys are zinc sulfide and strontium aluminate.
There are a couple of other types of luminescence. Chemiluminescence, for example, makes object glow in the dark because of a chemical reaction. When two particular chemicals react, they produce energy that is subsequently released, creating a glow. This is what happens in glow sticks.
Radioluminescence uses phosphors that are constantly charged by adding a radioactive element, such as radium, to them. You may have seen this type of luminescence on the hands of a watch, for example.
One final example from nature is bioluminescence. Some creatures, such as fireflies and jellyfish, contain chemicals within them that cause them to glow. Some of these creatures glow for protection, camouflage or to attract mates.
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All glow-in-the-dark products contain phosphors. A phosphor is a substance that radiates visible light after being energized. The two places where we most commonly see phosphors are in a TV screen or computer monitor and in fluorescent lights. In a TV screen, an electron beam strikes the phosphor to energize it (see How Television Works for details). In a fluorescent light, ultraviolet light energizes the phosphor. In both cases, what we see is visible light. A color TV screen actually contains thousands of tiny phosphor picture elements that emit three different colors (red, green and blue). In the case of a fluorescent light, there is normally a mixture of phosphors that together create light that looks white to us.
Take a phosphor that fits the bill, mix it in with the plastic to be molded into the product, and you have yourself a glow-in-the-dark whatever. Light from the sun or the living room lamp energizes the phosphors in the plastic and excites them, and with the lights off, you can watch as their atoms slowly lose this extra energy in the form of a dim glow.
Scientists have created numerous phosphors in the lab, but zinc sulfide and strontium aluminate are the ones that are most commonly used in glow-in-the-dark products, with strontium aluminate being the longer lasting of the two. The chemicals are mixed right in with the plastic that is molded into glow in the dark stars for your ceiling or added to the pigment of your Halloween make-up.
On rare occasions, something will glow in the dark without needing to be charged. These items still use phosphors to create the glow, but they add a radioactive element like radium to the compound. The radioactive element gives off small amounts of radiation, not enough to be dangerous, that constantly charge the phosphors in the same way a light would. Radiation-charged phosphors are typically used on clock or watch hands that need to glow hours after a light has been turned off.
Another way to make objects glow in the dark is through chemiluminescence, a chemical reaction. Two chemicals are mixed together, and the resulting reaction causes electrons to become excited, moving to a higher energy level. When the electrons return to normal levels, they release light energy, producing a glow. This is the type of reaction that is used to create the light in glow sticks.