Light Sound and Waves

Using ripple tanks

Teaching Guidance for 14-16 PRACTICAL PHYISCS

Students should whenever possible experience and experiment for themselves using real equipment, rather than using software which shows ripples. They need to try things out for themselves rather than just following instructions.

Practical tips: See apparatus note "Ripple tank and accessories" for important details.

Ripple tank and accessories

Asking questions – an activity which may help with discipline in a half-dark room – encourages students to think and extend their observations. When you ask whether the water moves along with the pattern, you could leave the students to devise their own tests and to think and experiment on their own, rather than giving detailed instructions.

It is worth considering where the dark and bright ripples come from. The convex and concave surfaces on the top of the wave make perfect lenses. When the light falls on the surface in the ripple tank then light is either focused by a convex surface or spread out by a concave surface. The concentrated light produces bright bands.

It takes time to set up ripple tanks properly. If you are going to use a set of ripple tanks for a class experiment, you may want to leave them on a side bench between successive lessons. Or, if the lesson follows lunch or morning break, you could ask a few students to come early and help set them up.

For demonstration purposes, you can now use a compact ripple tank designed to sit on an overhead projector. This produces a large image on screen, which the whole class will easily see.

Safety notes

Beware of water on the laboratory floor. Make sure you have a sponge and bucket handy to mop up spills immediately.

Place the power supply for the lamp on a bench, not on the floor by the tank.

Photo-induced epilepsy:

In all work with flashing lights, teachers must be aware of any student suffering from photo-induced epilepsy. This condition is very rare. However, make sensitive inquiry of any known epileptic to see whether an attack has ever been associated with flashing lights. If so, the student could be invited to leave the lab or shield his/her eyes as deemed advisable. It is impracticable to avoid the hazardous frequency range (7 to 15 Hz) in these experiments. The danger is obviously greater for xenon stroboscopes than for hand ones.

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