Counting photons one by one

What the Activity is for

This activity is all about hearing photons arrive, one by one. Draw out the random nature of their arrival, and the fact that the distance between the source and the detector seems to affect that rate.

What to Prepare

• a Geiger–Müller tube, connected to a counter with an audible output
• a gamma source
• some absorbers (thin sheets of lead and aluminium are suitable)

Safety note: This activity uses a radioactive source, and these should not be handled by students under 16 years of age.

What Happens During this Activity

Switch on the Geiger–Müller tube, and connect the counter, introducing it as a photon counter. Then introduce the gamma source as a photon source. Draw out some expectations about the detected count rate – what you hear – as the source is moved closer to or farther away from the detector. You might support the development of these expectations by asking whether the source looks more like a beam, more like a point source or somewhere in between. In any case you will find that the count rate drops with increasing separation between the detector and the source. This corresponds very nicely with the drop in brightness as you move farther and farther away from a light source. You might choose to have a light source to hand, precisely to draw out this connection.

If you have such a light source, you could start the next part of the experiment by placing an absorber in front of it (layers of tracing paper or greaseproof paper are probably about right). Draw out from the class what they'd expect to hear from your photon counter if you now place appropriate absorbers in front of the photon source. You'll need to be careful here to draw out the analogies in such a way that makes it obvious that we are dealing with two kinds of photons: those corresponding to optical frequencies and those corresponding to much higher frequencies, that we cannot see, but that our special photon counter can see.

You can extend the experiment further by comparing thicknesses of lead with thicknesses of photocopy paper, and thicknesses of aluminium with thicknesses of tracing or greaseproof paper. You'll need to practise to make this convincing. Do make the connection with the action of an optical filter, by linking the action of the additional sheets to the absorption of a fraction of the photons. Each sheet of absorber removes a certain proportion of the photons from the beam.