Two-Slit Interference
Light, Sound and Waves

Young's slits

for 14-16

You can demonstrate that light, sound, waves in water (ripples) and microwaves all produce the same interference pattern, from which wavelengths can be deduced.

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Young's slits with light

Two-Slit Interference
Light, Sound and Waves

Young's slits with light

Practical Activity for 14-16

Class practical

Two overlapping patches of light make bright and dark bands, giving direct evidence for the wave nature of light.

Apparatus and Materials

  • Graticule, ½  mm or Ronchi ruling
  • Lens (+14D)
  • Greaseproof paper screens, small
  • Power supply for lamp
  • Metre rule
  • Lamp, holder and stands
  • Setting up Young's experiment with light


  • Double slit, e.g. made from glass microscope slide
  • Double slits holder

Health & Safety and Technical Notes

Read our standard health & safety guidance


The lamp should be 48 W or 36 W, otherwise the fringes will be dim and hardly visible. Screen the lamp so that light is directed only towards the slits, and does not flood the room.

The whole point of this experiment is to see the pattern of fringes clearly, something that needs skill to arrange. See apparatus section on 'Setting up Young’s experiment with light' for details of how to make the interference pattern visible, and how to make the double slits.

Procedure

  1. Setting up. Allow at least a metre between the lamp filament and the slits, and several metres from the slits to the screen. A darkened room is essential. The slits must be parallel to the filament of the lamp.
  2. Viewing. Get students to look at the fringes with the naked eye, from behind the screen. It may be necessary to remind them to pull their heads back to a reasonable distance from the screen. Seeing the fringes – ‘light + light’ making ‘more light’ in some places but making no light in other places – is most important.
  3. Measurements. First measure the fringe spacing, x, by averaging over many. It may be best to mark the screen in pencil when the pattern is visible and later, in daylight, measure the spacing between the marks. Measure the distance from the slits to the screen, D, with a metre rule. Finally, measure the separation of the slits, s, (e.g. by comparing the slits with a calibrated graticule, using a magnifying glass to help).
  4. Estimating the wavelength. Lead the students through the geometry – see note 2 below – and then ask them to work out a very rough estimate of the wavelength.

Teaching Notes

  • Students see for themselves that two overlapping patches of light can make bright and dark bands, giving evidence for the wave nature of light. They can then go on to estimate the wavelength of light. Be careful that the second aim does not override the first.
  • Geometry.
    If the central band is at P and the next bright band at Q, the path difference, S1 Q – S2 Q must be one wavelength. Draw S2 M perpendicular to TQ. Then S1 M is the path difference, single wavelength. With the big distances and small angles involved, the triangle S1S2 M will be similar to PQT. Then, by similar triangles:

    wavelengthS1S2 = PQTQ

    wavelength =distance between slits  ×  fringe separationdistance from slits to screen

  • It’s worth explaining to students that making a rough estimate is often very good science. Progress in physics does not always consist of measuring one more decimal place with great precision. Here, a rough guess at the tiny wavelength of light is worth a great deal because it tells us why light seems to cast sharp shadows; yet it warns us that wave effects will become important when we go into fine detail.
  • Using white light, students will be estimating an average wavelength for the visible spectrum. The human eye is most sensitive in the green region, and the rough estimate is likely to be near to a value for green light, say about 5 × 10-7 m. You may want to interpose red and green colour filters. Switching quickly between them will make the fringes seem to grow and shrink, demonstrating the difference in spacing.
  • Lasers are very easy to use as the light source, but it would be a pity of students went away thinking that a laser is essential to see this effect.
  • You can two sets of semi-circular lines with an OHT to simulate the interference of coherent waves from two point sources.
  • get the sets


This experiment was safety-tested in January 2007

Up next

Young's fringes with sound waves

Two-Slit Interference
Light, Sound and Waves

Young's fringes with sound waves

Practical Activity for 14-16

Demonstration

This kinesthetic experience reinforces understanding of the interference pattern that results from two coherent sources producing circular waves. Compare this with a similar ripple tank experiment.

Apparatus and Materials

  • Signal generator, low frequency
  • Loudspeakers, 2

Health & Safety and Technical Notes

Read our standard health & safety guidance


This experiment is best done in the open air, with any reflecting buildings behind the loudspeakers. If the demonstration has to be done indoors, it helps to keep the sound as quiet as possible so that the reflected sound is not too obtrusive.

Set up the loudspeakers about 50 cm apart, if possible at shoulder height. Connect them to the output of the signal generator and set the frequency at about 2000 Hz. If you are doing this is indoors, you may need to adjust the frequency to achieve optimal results for the room size and surface reflections. A carpetted room with curtains round the walls (e.g. a stage) would be ideal.

If the central position turns out to be a minimum, reverse the connections to one of the speakers. This will make the central position a maximum.

A video showing the use of an signal generator is freely available at the National STEM Centre eLibrary.:

watch video


Procedure

  1. Turn on the signal generator. Get the students to walk across the area in front of the loudspeakers and listen for loud and quiet places. It may help them if they block one ear with a finger.
  2. Get students to stand where the sound is loudest. They should discover they are standing on hyperbolae with the two loudspeakers as foci.
  3. Now have students move to positions where the sound is a minimum. Cover one loudspeaker with something absorbent (for example, a cushion). Students should notice that the sound immediately gets louder.

Teaching Notes

  • If students have already seen this effect in a ripple tank, encourage them to visualize where they are ‘in the tank’. This will strongly reinforce understanding of the interference pattern. The hyperbolic nodal lines can be mapped out.
  • In experiment 3, it is better not to switch off one of the loudspeakers, as the power to the other speaker might change.
  • It is also possible to move a directional microphone connected to a CRO along a line parallel to the plane of the speakers and so see the maxima and minima on the screen.
  • A variation on the experiment which works well in a classroom: Mount two small speakers facing the same way at opposite ends of a stout piece of timber, about 2 m long. Connect the speakers in phase to the signal generator set at between 200 and 250 Hz. Grip the handle, point the speakers at the audience, and slowly rotate the supporting beam so that the interference pattern is swept across the audience.
  • You can follow this up using tuning forks. Hold vertically close to the ear and slowly rotate – you will clearly hear the increase and decrease in the sound level, as each prong acts as a source.
  • Sound is a longitudinal wave and this experiment reinforces the fact that longitudinal waves interfere. Interference diagrams normally imply transverse waves.
  • 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

Young's fringes with centimetre waves

Two-Slit Interference
Light, Sound and Waves

Young's fringes with centimetre waves

Practical Activity for 14-16

Demonstration

Shows that an invisible part of the electromagnetic spectrum too produces interference fringes from two coherent sources.

Apparatus and Materials

  • Microwave transmitter
  • Microwave receiver
  • Amplifier
  • Loudspeaker
  • Aluminium barriers

Health & Safety and Technical Notes

Modern equipment using a solid-state diode transmitter is safe. Older equipment using a klystron tube uses hazardous voltages. The connectors on the leads between the transmitter and the power supply MUST be shielded types to minimize the risk of serious electric shock. The ventilation holes in the power supply may also give access to hazardous voltages, so its use MUST be closely supervised.

Read our standard health & safety guidance


The microwave transmitter should be about 3 cm.

If the microwaves are unmodulated, simply connect the receiver to a meter. If they are modulated, it is possible to detect them using an amplifier and loudspeaker attached to the receiver.

Set up the transmitter symmetrically behind the two gaps, each of which is a half-wavelength wide.

Procedure

  1. Move the receiver in an arc, always the same distance from the gaps. The current through the meter should rise and fall to show about five maxima.
  2. Show that, when either gap is covered with a metal plate, the received signal decreases if the receiver is at a maximum but increases if the receiver is at a minimum.
  3. Photograph courtesy of Mike Vetterlein

Teaching Notes

  • If there is no risk of students thinking that the sound waves they hear are interfering with each other, it is worth modulating the transmitter and connecting the receiver to an amplifier and loudspeaker. The system then shows the change very clearly.
  • If you have a probe receiver (‘stick aerial’), use it rather than a horn receiver and you will get better results.
  • This might be an opportunity to discuss interference of radio and TV signals, when the direct signal interferes with its reflection from a building or an aircraft.
  • 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

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

Interference of light through two narrow holes

Two-Slit Interference
Light, Sound and Waves

Interference of light through two narrow holes

Practical Activity for 14-16

Demonstration

This is a simple demonstration of the interference of the light. From this, students can calculate the wavelength of the light source.

Apparatus and Materials

  • diode laser (Class 2)
  • wooden stand with clamp, 2
  • aluminium foil, 0.1 mm thicknewss, 5 cm x 5 cm
  • metre scale
  • sharp needle
  • travelling microscope
  • screen - A4 graph paper pasted on cardboard

Health & Safety and Technical Notes

Cheap laser pointers or toys cannot be recommended for use in UK schools. Only class 2 types are suitable.

Read our standard health & safety guidance


Make two narrow, very close circular holes in the aluminium foil by pressing the needle tip gently into the foil on a smooth plane surface.

Set up the apparatus, as shown in the diagram. The laser source, aluminium foil and screen should all lie in a straight line.

Procedure

  1. Switch on the source and make fine adjustments so that two holes in the foil are illuminated equally.
  2. Move the screen about 1 metre from the aluminium foil, so that the interference pattern falls on the graph paper. You should get a pattern of alternate dark and bright lines, of equal width and equal intensity, within a circular spot of light (see diagram above).
  3. Measure and record the average fringe width (β), with the help of graph paper markings.
  4. Measure and record the distance between the foil and the screen ( D ).
  5. Using a travelling microscope, measure the distance between the centres of the holes, x (see diagram).
  6. Calculate the wavelength of the light, λ, using the formula λ = X β/ D .

Teaching Notes

  • Derivation of the formula:
  • Let S1 and S2 be the centres of the holes acting like coherent sources.
  • Secondary wave fronts coming from these points undergo superposition (interference), leading to bright and dark fringes on the screen.
  • The ray diagram is shown above.
  • S2N is the path difference between S1P and S2P, where P is the position of a bright fringe.
  • The condition for constructive interferences gives S2N = nλ, where n is an integer.
  • In triangle S1NS2, sinθ = S2N/S1S2 = nλ/X
  • From triangle PRO, since θ is a small angle, sinθ = PR /OP ≈ Y/OR = Y/D
  • From the above relations, nλ/X = Y/D or Y = nλD/X
  • Let Y1 and Y2 be the distances of two consecutive bright fringes, 1st and 2nd fringes.
  • i.e. Y1 = λD/s , Y2 = 2λD/s
  • Therefore fringe width = Y2-Y1= λD/X
  • Or λ = BX /D
  • Experimental results conducted at physics laboratory CMRIT BANGALORE, using a Class 2 diode laser of wavelength 630 – 680 nm, 1 mW power:
  • For X = 0.046 cm,
  • D = 140 cm
  • 0.2 cm = fringe separation
  • the calculated λ is 657 nm.
  • For comparison, the result from a diffraction grating experiment was λ = 653 nm
  • The photo shows an interference pattern recorded with a 3.1 Megapixel digital camera.
  • 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 submitted by Tukaram Shet, Senior Lecturer in Physics at CMRIT Bangalore.

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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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