Reflection of ripples at a circular barrier

Demonstration

This experiment is best done after students have used a ripple tank to experiment with a pulse reflected by both straight and parabolic barriers.

Apparatus and Materials

For each group of students

• Ripple tank and accessories

• Barrier, semi-circular
• Wooden rod
• Water dropper

Health & Safety and Technical 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.

Read our standard health & safety guidance

Procedure

Ask students to try any experiments they like, using a circular barrier.

Teaching Notes

• Do not tell students to start a circular pulse from the centre of the reflector (but many will do that of their own accord). Do not tell them to find the place where straight pulses are brought to a point after reflection; and certainly do not ask them to measure that distance and see whether it is half the radius of the mirror. It is much better to leave students to their own experimenting.
• A ripple started at the centre of curvature of the barrier will return to the same point. A circular ripple started half way between the centre of the circular barrier and the barrier itself will produce a parallel beam. The connection with ray optics is clear.
• For very able students only – you might want to:
• discuss the geometry and ask students to locate a 'focus',
• ask students to turn the reflector round and use it as a convex reflector, and look for virtual image effects.

This experiment was safety-checked in February 2006