Diffraction at narrow openings

Class Practical

Using barriers in a ripple tank to see what happens to plane waves at narrow openings.

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)
• Barrier, short
• Straight barrier
• Moror mounted on beam, with beam support
• Side barriers (blocks of wood)
• Barrier, short

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

The short barrier should be about 2 cm long and weighted down so that it doesn’t float. A plastic object may be better than a metal one because it will scatter more clearly.

As the gap in the barrier is narrowed, until it is approximately the same size as the wavelength, then the emergent wave tends more and more to a semicircular shape. Some pupils will increase the gap width and see less diffraction with wider apertures.

Procedure

1. Set the barriers parallel to the vibrating beam and about 5 cm away from it, as shown. Make the gap between the barriers 2 cm or less.
2. Look carefully at the waves as they pass through the opening. You should see diffraction through almost 90° on each side.
3. If waves coming round the outside ends of the barriers are troublesome, block them off with side barriers.
4. Increase the frequency of the waves (making the wavelength shorter). How does the pattern change? [You may need to use a stroboscope to see the wave pattern clearly.]
5. Avoid using such high frequencies that the barriers themselves start to vibrate, giving misleading effects. You may want to try this with single pulses – the effects are easier to see.

Teaching Notes

• Diffraction experiments are best done before interference experiments. They show how a straight parallel wave can become a point source producing a circular wave at the gap in a barrier.

• As the gap in the barrier is narrowed, until it is approximately the same size as the wavelength, then the emergent wave tends more and more to a semicircular shape. Some pupils will increase the gap width and see less diffraction with wider apertures.
Additional investigations you might suggest that students try
• Generate circular ripples and see what happens as they pass through a gap.
• Place obstacles about 2 to 5 cm wide (e.g. the short barriers) near the vibrator. Students should see that long waves are scarcely affected but the ‘shadow’ of the obstacle becomes sharper as the wavelength is reduced (or as the size of the obstacle is increased). They will need to use a stroboscope.
• With a very small obstacle, some pupils may notice that weak circular ripples appear in the shadow as the main wave moves past otherwise undisturbed. This is just the way in which a small island affects sea waves.

This experiment was safety-tested in February 2006