Reflection at a parabolic barrier

Demonstration

Students may have done some ray optics with light using parabolic reflectors such as those in the back of car headlights. This ripple tank experiment helps to show how a parallel beam is produced.

Apparatus and Materials

• For each group of students

• Ripple tank and accessories

• Copper wire, heavy
• Wooden rod
• Rubber tube, heavy

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

The parabolic reflector can be made with rubber tubing. To help curve and anchor the tube, put heavy copper wire (or solder) into the tube before bending.

Procedure

1. First ask, 'What happens to straight line waves when they hit a parabolic reflecting wall?’
2. Whether or not students have done so before, get them to try this.
3. Then ask, ‘Can you turn that story backwards and make straight line waves come out from the wall?'
4. This is an exercise in thinking as a scientist, so we should be very careful not to reduce it to an exercise in carrying out instructions. Simply ask the question about the reverse effect.

Teaching Notes

• Students may have done some ray optics with light, using parabolic reflectors such as those in the back of car headlights. This experiment will help to show how a parallel beam is produced
• They should find the wave concentrated after reflection into a circular ripple which closed down to a small size and then spread out again.
• After students have seen the straight line ripple reflected into a circular ripple that moves to a point, they should know where to put their finger to start the reverse experiment.

This experiment was safety-tested in February 2006