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.
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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