Learning from spectra
Introductory and intermediate level courses
Spectra are beautiful things to see. The full spectrum of white light blazing across a screen is a surprise and a delight to students, even though they have seen a ‘dilute’ version often enough in a rainbow. The full spectrum produced from white light is described as a continuous spectrum, as every colour is present across a range of colours.
Absorption spectra are produced when light from a source is partially absorbed by passing through a medium, for example, a beam of white light passing through a green bottle. At intermediate level, colour filters are used to help students to understand the physics of colour. These too are generally continuous spectra.
The ‘single line’ spectrum of a salted flame (see the Carleton University website) can come as a surprise: instead of a broad band of yellow, students see a very narrow band of pure yellow, an emission spectrum. Passing white light through sodium vapour produces an absorption spectrum that has a narrow black line at exactly the same place in the yellow. With able students, you might pose the question, ‘how is it that some sources produce a continuous spectrum but others produce a line spectrum?’.
Advanced level courses
In advanced level courses, you can move on and explain why spectral lines are produced.
Energy levels in atoms
Historically, line spectra were among the earliest phenomena to give hints of energy levels and quantum behaviour in atoms, but the hint was not pursued by physicists until other phenomena pointed towards quanta, early in the 20th century. Scientists may now think of line spectra as offering clear evidence for energy levels. In the late 19th century, however, the phenomenon was still a great puzzle.
In advanced level courses, students can follow the argument from frequency differences through the quantum idea to energy levels. Colleagues in chemistry want students to know that atoms have well-defined, discrete energy levels. Amazing stability goes with that: atoms and molecules are completely elastic in collisions, up to a certain limit, above which they can store or release energy in discrete jumps. The experiments and reasoning which led to that knowledge are not directly relevant to its use in chemistry, and so this teaching in chemistry generally rests on simple assertion.
Electron interactions with atoms
Physicists study the stability of energy levels in atoms by experiments in which they bombard atoms of vapour or gas with electrons of known energy. Up to a certain kinetic energy, the bombarding electrons bounce off the target atom elastically. They give no energy to the atom, beyond the tiny share which is characteristic of the momentum exchange in an elastic collision. Bombarding electrons do not change the internal energy of the target atom at all. But above a certain minimum kinetic energy, bombarding electrons make an inelastic collision, giving a sharply defined amount of their kinetic energy to the target atom, which is changed to a higher state or energy level. That is shown by the Franck-Hertz experiment (the Wikiipedia website has information on this experiment) in which, originally, electrons bombarded mercury atoms in warm mercury vapour.
In a school laboratory, a similar experiment can be done with electrons bombarding an inert gas such as helium, using a Teltron tube. After inelastic collisions, the atoms of the target gas soon return to their ground state, emitting light as a spectral line. That links up well with a full study of line spectra.
Photon interactions with atoms
When photons bombard atoms, again there are contrasting cases of elastic and inelastic collisions, plain scattering of light and the Compton effect; and the various forms of the Raman effect. These too reveal discrete energy levels in atoms. They also suggest that radiation, from visible light to X-rays, carries its energy in quanta. A useful animation showing photon emission corresponding to transitions of electrons between atomic energy levels is shown on the BIGS website.
The atomic hydrogen spectrum can be shown with a grating. That spectrum, the Balmer series, has only four lines in the visible region, so students will not realize that the lines are part of a great series. Therefore, to supplement such measurements, you need to show a photograph (see this example on the Kyushu University website) of the Balmer series extending out into the ultra-violet. Those who like arithmetic puzzles might use the Balmer formula to see if their measurements fit.
Using spectral lines in astronomy
When astronomers look at spectra of distant stars and nebulae, they see spectral lines which obviously come from familiar elements studied in laboratories. These lines reveal the elements present in stars, interstellar space and galaxies.
In the spectra of remote galaxies, however, these lines are shifted towards the red. The red shift is greater for those galaxies which are further away (as judged by other evidence that seems trustworthy). A shift towards the red means a change to a longer wavelength, and also to a lower frequency. This suggests that galaxies are moving apart, and hence that space itself is expanding.