What it shows:
For an electron to make a transition from one energy level to a higher one, it needs to absorb a photon who's energy is equal to the difference in the energy levels involved. When jumping back down, it will emit a photon of that same energy. These discrete energy separations are characteristic of the atom involved, and it's what provides an atom with its fingerprint line spectrum. Trying to induce a transition with a photon of different energy just doesn't work.
In this demonstration, light from a sodium source will be absorbed by sodium gas, because the photon energies have the characteristic energies to cause a resonance transition. Radiation from a mercury source will not be absorbed however, because its emitted photons have frequencies that don't correspond to the sodium's energy levels.
How it works:
The demonstration employs three lamps; a sodium and mercury to provide the radiation, and a second sodium to provide an absorbing gas. All three lamps need to be heated up, which for the sodium takes about ten minutes, and the absorber lamp then turned off so that it is still hot and in a vapor state, but not being excited. Bring the mercury source in towards this lamp, and the tube remains completely clear and transparent. Now bring in the emitting sodium source; the incident radiation is absorbed, and the tube goes opaque and starts glowing the characteristic yellow.
Setting it up:
The demo requires two low pressure sodium lamps 1 and one mercury. 2 The absorbing sodium lamp rests horizontally (on a wooden cradle) with a black background, most easily provided by a velvet cloth. The emitting lamps need to be shielded from the audience because of the glare, and that can be provided by taping the lamp to a thin piece of plywood as long as and slightly wider than the lamp itself.
figure 1. The emitting lamp, shielded from the audience and the absorbing lamp upon a wooden cradle.
The back scattering from the absorbing sodium lamp is much stronger than the forward, so bringing in the radiant lamp from the front as shown in the diagram, is the best way to go. A very elegant and simple illustration of resonance absorption.
1 Phillips SOX 55W low pressure sodium lamp. North American Phillips Lighting Corp., Framingham, MA 01701
2 Vitalite 52 40W fluorescent lamp