Reflection at a parabolic barrier
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
Copper wire, heavy
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.
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.
First ask, ' What happens to straight line waves when they hit a parabolic reflecting wall?’
Whether or not students have done so before, get them to try this.
Then ask, ‘Can you turn that story backwards and make straight line waves come out from the wall?'
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.
1 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.
2 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-checked in February 2006