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Did you play with glow in the dark toys as a kid? They’re super cool, but kind of mysterious. How do they glow after all? Hank explains the phenomenon in this quick question.

Hosted by: Hank Green
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Zinc Sulfide:
Glow Sticks:
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QQs: How do toys glow in the dark?

Remember when glow in the dark toys were all the rage as a kid? There were bouncy balls and plastic bugs and those little green stars that everybody stuck on their ceilings. I totally did that. So you have to ask, as a member of the human species, how do they work?

Some TV commercials claimed magic but it's really thanks to these chemical compounds called phosphors. a phosphor is a substance that absorbs energy from another source - like a light for example - and then emits that energy as visible light, which is the glow we see.

There are natural phosphors in your teeth and fingernails, which is why they glow after absorbing energy from a black light. But chemists have also created lots of phosphors, and the most common ones in glow in the dark toys are zinc sulphide and strontium illuminate.

Zinc sulphide is really safe and affordable which makes it great for mixing into plastics to make glow in the dark toys. Strontium illuminate is more expensive, but it has a longer persistence, which means it emits visible light more slowly, and glows longer after being energized.

And both of these phosphors naturally emit greenish light - you know that pale iconic hue. But manufacturers can make other glow in the dark colors by adding different pigments to the plastic.

Because you energize these toys with light, whether it's the Sun or a lamp in your living room, we say that they're photoluminescent, but not all glow in the dark things work like that.

Glow sticks, for example, work by chemiluminescence which means that light is emitted as the product of a chemical reaction. When you crack a typical glow stick, you're allowing a hydrogen peroxide solution to react with diphenyl oxalate. This chain of chemical reactions releases energy, which is absorbed by electrons in fluorescent dye molecules that are also in the tube. That energy is emitted as photons of visible light, and it's a different color depending on the chemical structure of the dye molecules.

And some watches that glow in the dark work in a completely different way, through radioluminescence. In those, the phosphor is actually mixed with a radioactive element, like radium, tritium, or promethium that constantly energizes the phosphor to make it glow, without being dangerous to the humans using the watch.

But radioluminescence is rarely used nowadays because engineers are developing better photoluminescent compounds for watches as well.

So, if you're the proud owner of a bunch of glow in the dark stars or enjoy snapping some glow sticks on a dark night hopefully now you have a better idea of how they work.

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