Teaching sound at key stage 3 or 4 invariably involves the use of tuning forks.

Discussing how the prongs move to produce compaction and rarefaction of air molecules, thus producing the longitudinal waves that we hear as sound, can be awkward as the prongs cannot be seen to move.

Their movement can be detected by touching them to a glass or the surface of water. But I developed a way to show the movement directly during my teaching practice which involves a tuning fork and strobe light.

The strobe is set to about the frequency of the tuning fork - it is usually best to set the frequency before the class enter unless pupils are high ability in which case the process may interest them.

When the vibrating fork is held in front of the lamp and the flash frequency is gradually adjusted, the full movement of the prongs can be seen in apparently slow motion.

This also works if the strobe light is pointed towards the users so the tuning fork is backlit. The frequency for middle C is over 200Hz so there is no problem of using the lamp with pupils who are sensitive to flashing lights.

Alan Hedges is a supply teacher in Lancashire and Cumbria