What it shows:
Sodium 'D' line absorption showing up as a black line in the yellow of a continuous spectrum. Good as a simulation of the sodium portion of the Fraunhoffer absorption spectrum caused by atoms in the solar atmosphere; it does not however, resolve the 5890/5896Å doublet.
How it works:
As in the Sun, which is a black body source surrounded by an atmosphere of cooler gas containing many heavy atoms including sodium, we can set up a black body spectrum using a slide projector, and provide a hot sodium 'atmosphere' using a Bunsen and a spoonful of sodium bicarbonate. The burning sodium absorbs the incident light at its characteristic frequencies, re-radiating isotropically to leave the projected spectrum deficient in the characteristic sodium yellow.
Setting it up:
Use a Beseler Slide King projector with razor blade slit and focus onto a screen. Between the projector and screen place a grid-top Bunsen, then a direct view prism. The prism and Bunsen should be close, but still a safe distance apart for the prism’s sake! Put the Bunsen on a blue flame and sprinkle a teaspoon of sodium bicarbonate onto the grid. You can keep the absorption line present in the spectrum for some time by repeating this, and by tapping the Bunsen to ensure all the sodium bicarb has been burnt.
Dedicate a Bunsen to this demo, or your chemist will murder you when s/he goes to do a flame test. Be generous with the sodium bicarb - it's cheap, works better with a lot, and adds to the fun of a really good show.