Science Question from a Toddler: The color of light


Why does a glow-in-the-dark Frisbee glow green? Why does a spark from a light socket look blue? Two different questions, but one intertwined answer.

Hopefully, readers Inga Foster—who asked about electricity—and Stewart Haddock—the man with the glow-in-the-dark query—don't mind being lumped together. As it turned out, they were really asking about the same thing. Both these phenomena stem from the weird ways light interacts with atoms.

Yes, we're talking about physics today. But don't worry. If I can understand it, you can understand it.

In one corner, we have the atom. You know this guy. He's the basic building block of everything, everywhere. Tinier than tiny. But also very powerful.

Each atom has a nucleus—a ball of particles that carry positive and neutral electrical charges—and is circled by electrons, particles with a negative electrical charge.

In the other corner: Electromagnetic waves. What these waves do depends on their frequency—how fast they vibrate. High-frequency waves bring light to our eyes, and determine what colors we see. There's a range of frequencies that can produce visible light, and we perceive the different frequencies as different colors.

It goes on a gradient, like a rainbow. We see higher pitched waves as blue, lower pitched ones as red and the other colors fall somewhere in between. The waves can also be so high frequency or low frequency that our eyes can't see them at all, and that's where you get into things like ultraviolet and infrared light.

Now, say you're a little atom, just hanging out, minding your own business, when you're hit by some form of energy. You can absorb some of that energy, but not all of it.

"When the atom absorbs energy, the electrons become very energized, but electrons don't like to be over-stimulated. They like to be home, just like everybody likes to be where it's comfortable," said Andrew Glassner, Ph.D.

Glassner is a former research scientist who designed computer graphics algorithms to produce true-to-the-real-world simulations of lightning and glow-in-the-dark objects in the 1990s and 2000. To get the models to work correctly, he had to study the physics behind the phenomena and incorporate that into the algorithms.

"If you take an electron and make it very, very excited, it will try to shed that excitement and go home again. The energy has to go somewhere, though, and the way electrons get rid of energy is by spitting it out," he said.

The atom spits out electromagnetic waves of a specific frequency depending on its charge and mass—which means that different atomic elements have different characteristic colors.

"That's actually how we know what chemicals make up the sun. We can look at the sun and see what frequencies are coming off. We can say, 'Those colors come from helium. So, by golly, the sun must have some helium in it!,'" Glassner said.

Them's the basics. But how does this play out for electric sparks and Frisbees?

"With electric sparks, the color you're seeing is mostly nitrogen from the atmosphere," said Bill Beaty, a research engineer for the chemistry department at the University of Washington who's consulted on textbooks and museum science education programs for kids. "If the air was neon rather than nitrogen we'd think electricity was orange."

What we see as blue light from an electric spark is simply the result of nitrogen atoms absorbing electrical energy, and spitting some back out in the form of electromagnetic waves—waves which, to us, happen to appear blue.

In fact, electricity doesn't always appear blue. The center of a spark, and lightning, both appear white. That's because when you hit an atom with higher levels of energy it will release waves of several different frequencies. Our eyes perceive each frequency as a different color—and white is just the color we see when we see lots of colors merged together.

The Frisbee work much the same way. Zinc sulfide is a cheap, naturally occurring chemical compound. About a century ago, people realized that if you took zinc sulfide and exposed it to light energy it would absorb some of that light, but also, slowly, spit light back out over several hours.

The atoms that make up zinc sulfide happen to spit out their waves at an frequency that, to us, appears ghostly green.

There's lots of other natural and man-made chemicals—called phosphors—that will do this, and in different colors. Glow-in-the-dark can really be any color you want these days. But zinc sulfide was the one that was put on watch hands, exit signs and (yes) Frisbees for much of the 20th century. So, really, the reason we think of glow-in-the-dark as green is more of a cultural thing, than a fact of science.

Image courtesy Flickr user methticalman, via CC