Topic 13.6
Diffraction
Properties of microwaves
Aims
In this practical activity, you will measure the wavelength of microwaves and investigate
diffraction and interference using microwaves.
Safety
The power supply of the microwave transmitter is a high-voltage supply. Do not change the
connections or disconnect the microwave transmitter from its power supply.
Equipment and materials
For this experiment you will need a low-power microwave transmitter, a microwave receiver
connected to a suitable meter, a metre ruler, two square metal plates and a narrow metal plate
(all three plates on stands).
Part 1
Measuring the wavelength of microwaves
Method
1 Place a microwave detector probe between the microwave transmitter and the metal plate,
as shown in Figure 1. The metal plate reflects microwaves from the transmitter back
towards the detector. Move the detector along the line between the transmitter and the
plate and observe the effect on the detector signal. The microwaves reflected by the metal
plate and the microwaves travelling towards the metal plate form a stationary wave
pattern.
Figure 1 Measuring wavelength
2 The distance between adjacent nodes (that is, points of minimum intensity) is half a
wavelength. Make suitable measurements to determine the wavelength. Estimate the
uncertainty in your measurement. Record your measurements and your value of the
wavelength and its uncertainty.
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Topic 13.6
Diffraction
Questions
1 (a) Explain how you measured the wavelength of the microwaves.
(b) Describe how you estimated the uncertainty of the wavelength value.
Part 2
Diffraction of microwaves
Method
1 Use two metal plates to form a gap of width 35 mm. With the transmitter at a fixed
distance from the gap, aim the microwaves from the transmitter at the gap. Use the
detector to investigate the spread of microwaves passing through the gap.
2 Use a millimetre rule to position the detector at regular distances along a line parallel to
the plates on the other side of the plates to the transmitter. Use the meter to measure the
intensity of the microwaves at different positions along the line. Record your
measurements.
3 Make the gap twice as wide and repeat the measurements.
4 Plot your measurements as intensity–distance curves on the same axes.
Questions
2 (a) Discuss whether or not the intensity–distance curves shows the features expected of a
single slit diffraction curve.
(b) Describe and explain two differences between the two curves.
Part 3
Interference of microwaves
Method
1 Use a narrow metal plate and two wide metal plates to form adjacent narrow gaps.
Measure the distance between the gaps from centre to centre. Position the transmitter
exactly opposite the gaps, as shown in Figure 2.
Figure 2 Interference
2 Move the detector along a line parallel to the gaps and locate the positions of minimum
intensity. With the detector at such a position, observe the effect on the detector signal of
blocking each gap in turn. For each position of minimum intensity, measure the distance
to the centre of each gap.
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Topic 13.6
Diffraction
3 Make a scale drawing of the gaps and the points of minimum intensity.
Questions
3 (a) Describe and explain what you observed when the detector was at minimum intensity
and one of the gaps was then blocked.
(b) Show that the path difference to each position of minimum intensity from the centres
of the two gaps is an odd number of half wavelengths.
Teacher Notes
Aims
 To measure the wavelength of microwaves and to estimate the uncertainty in the measurement.
 To make measurements on the diffraction of microwaves and plot intensity–distance curves.
 To demonstrate interference of microwaves and plot the results on a chart.
Safety
The power supply of the microwave transmitter is a high-voltage supply. Tell the students not
to change the connections or disconnect the microwave transmitter from its power supply.
Teaching notes
4 In part 1, remind the students to measure across as many nodes as possible and to estimate
the uncertainty in this measurement. The distance between adjacent nodes and the
associated uncertainty can then be calculated to give the wavelength of the microwaves.
5 In part 2, the students should attempt to obtain a wide central diffraction ‘fringe’ in the
first set of measurements. By doubling the gap, they should then see that the central
diffraction fringe is much narrower.
6 Part 3 may be conducted as a group exercise in which each student of pair of students
locates a different minimum, makes measurements to that point from the centre of the slits
and then plots the point together with points obtained by others on a chart. Using the
wavelength measurement from part 1, they should see that the path difference from any
such point is always an odd number of half wavelengths.
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Topic 13.6
Diffraction
Answers to questions
1 (a) The answer should include some information about how many half wavelengths
made up the measured distance and how the wavelength was calculated.
(c) The students should describe how they located each of the two nodes that determined
the measured distance.
2 (a) The curves should show maximum intensity directly opposite the centre of the gap,
then a decreasing intensity to zero followed by increasing intensity, and so on, with
increasing distance from the centre.
(d) The curve for the wider gap should show an intensity distribution corresponding to a
narrower central fringe than the other gap and at least one outer fringe either side of
the centre half, which is the width of the central fringe.
3 (a) Blocking either one of the gaps would increase the intensity of the microwaves at the
receiver. This is because the microwaves from the unblocked gap are not cancelled
out by microwaves through the other gap because is has been blocked.
(e) The measurements should show the path difference is an odd number of half
wavelengths, taking into account the uncertainty in the value of the wavelength.
Practical assessment opportunities
4 PSA – Selecting and using equipment: 0–3 marks
(Following instructions and group work; organisation and safety assessed at end of
course.)
5 ISA preparation:
practical task – practice in making and recording measurements and plotting a graph and a
chart
written task Section A and B – opportunities to estimate and use uncertainties
Technicians Notes
Equipment and materials required
Each group or pair of students will require the following equipment:
 a low-power microwave transmitter
 a microwave receiver connected to a suitable meter
 a metre ruler
 two square metal plates and a narrow metal plate, all on stands
 graph paper
The power supply of the microwave transmitter is a high-voltage supply. Ensure the students
know not to change the connections or disconnect the microwave transmitter from its power
supply. Make sure connecting leads carrying mains voltage cannot be removed, or have
shrouded plugs.
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