Glow in the Dark


  • How glow in the dark things work, part 1 

If you’re a member of my generation or the one that raised it, your house was probably full of all sorts of glow-in-the-dark stuff in the 80s and 90s. Yo-yo’s, stickers, action figures, clothing, you name it. As a kid, I thought it was just short of magic. The effect is less impressive to adult me, but the chemistry behind it is pretty cool.

Your average glow-in-the-dark doodad gets its glow from a phosphor, a member of a group of substances that radiate visible light after being energized. Some phosphors are natural, like ones found in your teeth and fingernails, and chemists have also created hundreds of others. The ones most useful for glow-in-the-dark items are those that can be energized by normal light and have a pretty long persistence (glow time).

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.

Beyond the usual glow-in-the-dark artifact, there are some special cases where glowing products work a little differently. Glow sticks work by chemiluminescence — that is, the light is emitted as a product of a chemical reaction. Items that need to glow continuously with little or no “charge,” like clock or watch hands that glow for hours after a light has been turned off, work by radioluminescence. Timepieces like this still use phosphors to create the glow, but also have a little bit of a radioactive element like radium added to the glowing parts, which gives off small amounts of energy — not enough to be dangerous to the user, but, historically, a problem for the people who make the products — that constantly charge the phosphors in the same way a light would and keep the item glowing through the night. Find this wonderful article here – there is some brilliant information on the site.

Source: Mental Floss


  • How glow in the dark things work, part 2

You see glow-in-the-dark stuff in all kinds of places, but it is most common in toys. My son, for example, has a glow-in-the-dark yo-yo, a glow-in-the-dark ball, a glow-in-the-dark mobile and even (if you can believe it) a pair of glow-in-the-dark pajamas! They make him easy to find at night!

If you have ever seen any of these products, you know that they all have to be “charged”. You hold them up to a light, and then take them to a dark place. In the dark they will glow for 10 minutes. Some of the newer glow-in-the-dark stuff will glow for several hours. Usually it is a soft green light, and it is not very bright. You need to be in nearly complete darkness to notice it.

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.

Chemists have created thousands of chemical substances that behave like a phosphor. Phosphors have three characteristics:

  • The type of energy they require to be energized
  • The color of the visible light that they produce
  • The length of time that they glow after being energized (known as the persistence of the phosphor)

To make a glow-in-the-dark toy, what you want is a phosphor that is energized by normal light and that has a very long persistence. Two phosphors that have these properties are Zinc Sulfide and Strontium Aluminate. Strontium Aluminate is newer — it’s what you see in the “super” glow-in-the-dark toys. It has a much longer persistence than Zinc Sulfide does. The phosphor is mixed into a plastic and molded to make most glow-in-the-dark stuff.

Occasionally you will see something glowing but it does not need charging. The most common place is on the hands of expensive watches. In these products, the phosphor is mixed with a radioactive element, and the radioactive emissions (see How Nuclear Radiation Works) energize the phosphor continuously. In the past, the radioactive element was radium, which has a half-life of 1600 years. Today, most glowing watches use a radioactive isotope of hydrogen called tritium (which has a half-life of 12 years) or promethium, a man-made radioactive element with a half-life of around three years.

Source: How Stuff Works

  •  Do glow in the dark things cause cancer?

The radioactive glowing compounds used ionizing radiation to cause the glow. They did not need to be exposed to the light before starting to glow, and the glowing would not stop until the radioactive substance depletes. That same radiation can damage the DNA in your cells and cause cancer.

The first generation used radium, which emits alpha radiation. As you noted alpha particles cannot travel far enough to cause the damage to the skin through the thickness of a watch or even air. The problem with radium is that chemically it is similar to calcium, and the factory workers exposed to radium paints accumulated it in their bones instead of calcium, thereby having direct exposure to radioactivity, and suffered from cancer and other effects.

The next generation used promethium, which is a beta emitter. Beta particles are simply electrons, and are also used in all CRT screens that use electron guns to make the phosphor glow. Promethium’s beta particles can cause X-rays when interacting with certain substances, and is therefore a health hazard. It does not replace calcium in the bones, so is safer than radium in that respect, although the radiation is more dangerous.

The third generation used tritium (an isotope of hydrogen), which is also a beta emitter. Its kind of beta particles cannot cause X-rays, can travel only about 1/4 inch in air, and can’t penetrate beyond the thin dead skin layer. This is the safest permanently glowing technology.

The modern glow in the dark compounds use an entirely different principle. They have molecules that can be excited to an energized state by the visible light and then due to the laws of quantum mechanics get stuck in such state for a long time. As they after a while come down from this energized state to the ground (lowest energy) one they emit the excess energy as visible light. There is no ionizing radiation involved, so if you don’t ingest or inhale such substances they are safe for handling in the longest term. The drawback is that the need to be charged in the light, as there is no internal source of energy for glowing.

The short version of this seems to be: Not the modern variety but some of the older ones used the element Radium to generate the light and Radium is radioactive and can cause cancer.


Author: Janet Carr

Fashion, beauty and animal loving language consultant from South Africa living in Stockholm, Sweden.

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