At the end of 19th century, physicists knew there were electrons inside atoms, and that the wiggling of these electrons gave off light and other electromagnetic radiation. But there was still a curious mystery to solve. Physicists would heat up different elements until they glowed, and then direct the light through a prism...

I've done that with sunlight. You see the whole rainbow because the prism breaks the light into all of its separate colors.

That's what you get with light from the sun. But when scientists looked at the light coming off of just one element, hydrogen for instance, they didn't see the whole rainbow. Instead they just got bright lines of certain colors. (Actually, "color" isn't the right term, because only some of the lines were visible, but for now we'll just talk about visible light.)

That would mean that the atoms were only emitting waves of certain frequencies. Do all atoms create the same colors?

No. Each type of atom gives off a unique set of colors. The colored lines (or Spectral Lines) are a kind of "signature" for the atoms.

Kind of like wearing your team colors.

Team Oxygen

Team Carbon

Exactly. If you put light from a common streetlamp through a prism, or look at the light through a diffraction grating, you will see distinct lines. Two common kinds of street lights use sodium vapor and mercury vapor bulbs. Each of these lights has a different spectral "signature," and you can tell what kind of lamp it is by its spectral lines.

Pick an element from the menu to see its spectral signature.

Is that why different street lights seem to be different colors?

You got it. This technique is so reliable that scientists can tell what elements they are looking at just by reading the lines. Spectroscopy is the science of using spectral lines to figure out what something is made of. That's how we know the composition of distant stars, for instance.

Wait a second. We learned earlier that radiation is caused by wiggling charges, and the rate of the wiggling determines the wavelength. If only some wavelengths are coming out of the atom, that would mean that the electrons are wiggling at only some frequencies. How does that happen?

That was the big puzzle. Fortunately, a Danish physicist named Niels Bohr came up with an answer...

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