Dear Haygarth,
I love glow-in-the-dark stickers. I leave them next to a sunny window for a little while. Then turn off the lights and watch them glow.
I asked my friend Hergen Eilers how those stickers work. He’s a physicist at Washington State University.
He told me that glow-in-the-dark things use a type of phosphorescence. That means they absorb energy—like light—and then glow. A material that can do that is called a phosphor.
Glowing in the dark works because of electrons.
Everything in the universe is made of atoms. That includes you, me, whatever you’re reading this note on—and glow-in-the-dark stickers.
All those atoms are made of protons, neutrons and electrons. Protons and neutrons stick together in the middle of the atom. Electrons make a cloud around them.
Sometimes we say electrons are in orbitals. That’s a way to describe where an electron could be and how much energy it has.
Electrons with lots of energy are farther away from the middle of the atom. They’re in a higher orbital. Electrons with less energy are closer to the middle. They’re in a lower orbital.
Things glow-in-the-dark because electrons can move to different orbitals.
“Light of relatively high energy comes in,” Eilers said. “One of the electrons absorbs it and is kicked into a higher orbit. Typically, it then falls back pretty rapidly and emits some light.”
So, light hits a phosphor—a glow-in-the-dark thing. The electrons in the phosphor suck in the light’s energy. All that extra energy makes those electrons zoom to higher orbitals.
Then, the electrons burp out the extra energy as light. They fall back down to their usual orbital.
If it happened just like that, the glow of the burped-out light wouldn’t be very exciting. It would happen too fast. But phosphors aren’t perfect. Nothing in nature is perfect.
The tiny flaws in a phosphor—like missing atoms—make little nooks or holes. When electrons zoom up to higher orbitals, they can get trapped in those holes.
“Sooner or later—sometimes minutes, sometimes hours or even days—the trapped electrons come back down,” Eilers said. “That’s what you’re seeing in the dark.”
My glow-in-the-dark stickers glow because electrons in the sticker have gotten stuck in itty bitty flaws that act like traps. Then the electrons let out the extra energy and fall back through the traps to their normal places.
Without those flaws, glow-in-the-dark stuff wouldn’t work. I guess cool things happen because nature is an imperfect glow-getter.
Sincerely,
Dr. Universe