Single-Slit Diffraction
Light, Sound and Waves

Diffraction in a ripple tank

for 14-16

This collection begins with diffraction at a single opening, then goes on to investigate a double gap and finally multiple gaps. These experiments could be used to explain diffraction gratings with light as well as Young’s double slit experiment. 

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Diffraction at wide openings

Single-Slit Diffraction
Light, Sound and Waves

Diffraction at wide openings

Practical Activity for 14-16

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

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

Up next

Diffraction at narrow openings

Single-Slit Diffraction
Light, Sound and Waves

Diffraction at narrow openings

Practical Activity for 14-16

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

Up next

Interference of water waves from two gaps

Two-Slit Interference
Light, Sound and Waves

Interference of water waves from two gaps

Practical Activity for 14-16

Demonstration

Plane waves in a ripple tank strike two narrow gaps. Each gap produces circular waves beyond the barriers, and the result is an interference pattern.

Apparatus and Materials

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 side barriers prevent stray edge effects.

Procedure

  1. Set up two straight barriers with a short one between them, along a line parallel to the beam and 10 cm to 15 cm away from it, as shown in the diagram. Make the gaps between barriers about 1 cm wide.
  2. Generate straight waves with a wavelength of about 1 cm.
  3. Draw students’ attention to the pattern made after the waves emerge from the gaps. If they need to freeze the pattern, offer hand stroboscopes.

Teaching Notes

  • A straight, parallel wave striking the two narrow gaps changes into two circular waves and interference effects are seen beyond the barrier. The pattern is the same as would be produced by two dippers vibrating in phase at the gaps, but fainter. The waves emanating from the two gaps carry less energy than waves produced by two dippers. Once students accept that each gap produces waves like those of a feebly moving dipper, they know what pattern to look for.
  • This experiment demonstrates the technique that Young used to show the interference of light. By dividing a single wavefront, he hit upon a way of generating two point sources of waves that remain in phase with each other.
  • Note: The nodal lines will spread out along hyperbolic paths and so the simple Young's fringes formula will not apply.
  • fringe separation = wavelength x distance from vibrator / slit separation
  • This...

    template


    ...with two sets of semi-circular lines can be used with an OHT to simulate the interference of coherent waves from two point sources.

This experiment was safety-tested in January 2007

Up next

Diffraction of a plane wave by multiple gaps

Single-Slit Diffraction
Light, Sound and Waves

Diffraction of a plane wave by multiple gaps

Practical Activity for 14-16

Demonstration

This ripple tank experiment models the action of a diffraction grating.

Apparatus and Materials

  • Ripple tank and accessories


  • Barriers, small
  • Barriers, large
  • Power supply, low voltage Continuously variable
  • Motor mounted on beam, with beam support
  • Light source, compact

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


You need 2 large barriers and 6 small barriers.

Avoid very high motor speeds, which cause unwanted vibration of the barriers.

Procedure

  1. First set up the ripple tank with about 1 cm depth of water.
  2. Set up a line of small barriers 5 cm from the vibrator, as shown. There should be a gap of 2 to 3 cm between each.
  3. Start the motor at a low speed (4 rev/second).
  4. Ask: "Can you see semicircular ripples emerging from the gaps? Further out, can you see waves moving out in slanting directions, as well as a wave moving straight ahead?"
  5. Students should observe the interference pattern carefully, with and without stroboscopes.
  6. Keeping the barriers arrangement the same, gradually increase the motor speed.
  7. Ask: "How does the interference pattern change?"

Teaching Notes

  • The pattern produced with multiple gaps is less clear than the double gaps experiment but, with care in aligning the gaps, it is visible. It will help if students have first seen diagrams of sets of semicircles to represent a snapshot of waves proceeding from several gaps.
  • The spacing of nodal lines will decrease as the wave frequency increases.
  • If students agree that the many gaps ‘grating’ produces circular waves in various directions, you can show them some simple geometry. Draw ‘rays’ from successive gaps to a diffracted plane wave-front and point out that the extra path when you change from one gap to the next is one wavelength for first order. Then the geometry is clear.
  • one wavelength / d = sin A
  • where d is the slit separation and A is the angle of deviation of the new wavefront from the direction of original beam.
  • You may want to make a direct comparison between this experiment and light striking a diffraction grating. If a beam of white light is a stream of waves, a wavelet must emerge from every illuminated gap of the grating. Since the gaps must be very narrow — because there are so many crowded together in the small grating – those wavelets must almost be semicircles. Look at them in the diagram and in the ripple tank.
  • same path lengths
  • If these wavelets continue out to a screen very far away, there will be places where the wavelets from all the gaps arrive in step. In the central bright image all those contributions have the same path length - they arrive in step whatever their wavelength, so the central image is white.
  • path difference of one wavelength
  • In certain directions to either side of the direct image, contributions arrive in step because the path difference from one gap is just one wavelength more than the path from its neighbour.

This experiment was safety-tested in February 2006

Up next

Using wave simulations

Interference
Light Sound and Waves

Using wave simulations

Teaching Guidance for 14-16

There are many excellent applets available online that show wave behaviour as if observing a ripple tank or oscilloscope screen.

These cannot substitute for experience of the phenomena themselves but provide a powerful way of helping students to visualize. They provide a valuable complement to experiments by removing extraneous effects.

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