Physics 2000 Science Trek Quantum Atom

Atomic Spectra

We've seen how photons are created or destroyed when interacting with atoms, but an important thing to realize is that transitions between the same energy levels always produce the same color photon. (Actually, photons don't have colors but often that is a convenient way of thinking about their wavelength or frequency.)

I can show you how to compute the energy (and thus the frequency and wavelength) of these photons...

In the following experiment, there is a device below the Bohr model that works like a prism or a diffraction grating. It shows the atomic spectrum for a hydrogen atom. Whenever a photon is emitted, it shows up on the spectrum according to its wavelength.

Click on an orbit to make the electron jump energy levels.

Huh. Each time the electron jumps down a level it produces a photon, and the same jumps produce the same colors.

When you have a whole lot of atoms, I'll bet you get all these different lines appearing at once.

Exactly, and that's what scientists mean by the atomic spectrum. By the way, the converse is true, too. Those same color photons are the only ones that will bump the electron up to higher levels. Photons of other frequency will pass right through the atom.

That would mean atoms are kind of "transparent" to all light except their own "team colors."

We keep talking about the "color" of these photons. Does that mean that atoms only interact with visible light? What about other kinds of electromagnetic radiation?

We've been talking about visible light because it's the easiest to experiment with. But you're right, we should talk about the "frequency" or "wavelength" of the photons, not their color. In fact, we're now going to talk about how heavier atoms, which have lots of electrons, tend to interact with higher energy waves, like x-rays. We can go ahead and talk about these heavier atoms, or look at some specific examples, such as how hospital x-ray machines create the x-rays, or how they absorb them to make images.

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