Sorting Spectra
OBJECTIVE
Students will compare stellar spectra for patterns and sort accordingly. Students
will compare their observations with the class.
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.
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.)
Emission Spectra
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.
Procedure:
1. Each 2 students will receive a set of spectra.
2. Look at the spectra and organize them into 4 or more groups.
3. At your table, you will discuss with the other 2 student groups why you chose
the categories you did and explain them. Why did you organize the spectra in the
way you did? What was your criteria or rational for organization?
You have about 10 minutes to organize your spectra.
You have about 10-15 minutes for discussion