UTF-8Recording the photons with a spectroscope255255204
Say you've got a gas composed of a bunch of atoms of different kinds and you want to know what's in it. Since,as we've seen, every atom emits photons of distinct frequencies, all you would need is a table of those frequencies for all the different types of atoms, and a way of measuring frequency of photons. Then all you'd have to do is heat up the gas, measure the photons it emits, and match the frequencies against your table, right?
Well, guess what? Tables of photon frequencies (they're called spectra) are available for every type of atom in the universe. And it's childlishly easy to measure the frequency of a photon. Remember, the frequency of a photon is related to its color -- low frequencies correspond to red and orange, high frequencies are blue and violet, yellow and green are in between. So all you have to do is separate the photons by color, say with a prism, and take a picture! There is a special instrument, in fact, that does this -- it's called a "spectrograph" because is produces spectra (the photon colors from different atoms).
If you feed a spectrograph plain old white light, which contains all the colors in the rainbow, it produces exactly what you would expect -- a rainbow. Here's what that looks like:
[Picture of an absorption spectrum with no lines in it]
We've attached one of these spectrographs to the model below. Run the model and it will capture all the photons emitted by the purple atoms and produce the spectrum.
Note: the spectrum should have frequency values running along the bottom.
Do you notice anything strange about the spectrum? First of all, it doesn't look anything like a rainbow, does it? That's because we didn't start with white light. The light from the purple atoms is composed of just three frequencies, as we have already discovered, corresponding to transitions between the three energy levels. The three vertical lines in the spectrum are tiny segments of the overall rainbow of frequencies. They are called "spectral lines."
The other strange thing you might have noticed is that only one of the spectral lines of the purple atom, the bluish one in the middle, is an actual color, the other two are colored white. That's because the photons that created them don't actually have a visible color. The low frequency one, the one on the left, is infrared. The line on the right is the high frequency one. It's ultraviolet.
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