Physics 2000 Science Trek Quantum Atom

Absorption Spectra

Like many important scientific discoveries, Fraunhofer's observation of spectral lines was a complete accident. Fraunhofer wasn't looking for anything of the sort; he was simply testing some new state-of-the-art prisms he had made. When sunlight was sent through a thin slit and then through one of the prisms, it formed a rainbow-colored spectrum, just as Fraunhofer had expected--but, much to his surprise, the spectrum contained a series of dark lines.

Dark lines? That's the opposite of what we've been talking about. You've been telling me that different elements create a series of bright lines at certain wavelengths.

That's what happens when an element is heated. In terms of the Bohr model, heating the atoms gives them some extra energy, so some of their electrons can jump up to higher energy levels. Then, when one of these electrons drops back down to a lower level, it emits a photon --at one of that element's special frequencies, of course.

And those photons create the bright lines in the spectra you showed me.

Exactly--that's called an emission spectrum. But there is another way in which elements can produce spectra. Suppose that instead of a heated sample of some element, you have the element in the form of a relatively cool gas. Now let's say that a source of white light-- containing all visible wavelengths--is shining behind the gas. When photons from the light source make their way through this gas, some of them can interact with the atoms--provided that they have just the right frequency to bump an electron of that element up to a higher energy level. Photons at those particular frequencies are thus absorbed by the gas. However, as you noted before, the atoms are "transparent" to photons of other frequencies...

So all those other frequencies would come through okay. Then the spectrum of light that had been through the gas would just have some gaps in it, at the frequencies that were absorbed.

That's right. The spectrum with these missing frequencies is called an absorption spectrum. (Note that the dark lines in an absorption spectrum appear at exactly the same frequencies as the bright lines in the corresponding emission spectrum.)

And that's what Fraunhofer saw?

Yes. Under very careful examination, the "continuous" spectrum of sunlight turns out to be an absorption spectrum. In order to reach earth, sunlight needs to pass through the sun's atmosphere, which is a lot cooler than the part of the sun where light is emitted. Gases in the atmosphere thus absorb certain frequencies, creating the 600 or so dark lines that Fraunhofer observed. (They are now called Fraunhofer lines in his honor.)

Fraunhofer was unaware of all this, however. No one offered an explanation of spectral lines until decades later...

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