Diffraction at wide openings

Class Practical

Using barriers in a ripple tank to see what happens to plane waves at wide openings and edges.

Apparatus and Materials

For each student group

• Ripple tank and accessories

• Hand stroboscope

• Motor mounted on beam, with beam support
• Side barriers (blocks of wood)
• Straight barrier, 2

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.

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.

Read our standard health & safety guidance

Procedure

1. As shown in the diagram, set the two straight barriers parallel to the vibrating beam and about 5 cm away from it. Make the gap between the barriers about 10 cm.
2. Generate straight waves with wavelength of about 1 cm.
3. Observe carefully what happens as the waves pass through the opening. You should see only a little spreading at the edges, called diffraction.
4. waves coming round the outside ends of the barriers are troublesome, block them off with side barriers.

Teaching Notes

• You may want to instruct your students to begin with only one side of the barrier and so see some evidence of diffraction effects from a single edge. They can then bring up the second half of the barrier from a distance and keep the gap wide.
• Some students will alter the gap width and see more diffraction with narrower apertures.

This experiment was safety-tested in February 2006