Topic 13.4
Double slit interference
Two source interference
Aims
In this practical you will observe a light beam split into two sources by a double slit. You will
then observe the interference pattern created and take measurements in order to calculate the
wavelength of the beam. This will enable you to recognise the appearance of the interference
fringes produced by a double slit system.
Safety
 Check all wiring for loose connections.
 Ensure that all bags are safely stored under desks or in cupboards.
 The lamp in the ray box will get hot.
Equipment, materials and method
1 For this experiment you will need a ray box and a low-voltage supply, a narrow slit, a
white paper screen, a millimetre ruler, a metre ruler and a double slit. You will need to
carry out the experiment in a darkened room.
2 Position the ray box about 1.0 m from the screen. Support the double slit directly in front
of the beam near the ray box. Observe the interference fringes on the screen. The slit in
the ray box needs to be as narrow as possible to make the fringes distinct but wide enough
so that the beam covers the double slits.
3 Measure the distance between interference fringes. Repeat the measurement for as many
fringes as possible and find the average fringe spacing w.
4 Switch off the ray box and measure the exact distance D from the double slit to the screen.
5 Measure the separation of the slits, s, if this has not already been done.
Question
What would happen if one of the slits were covered over?
Results
Use your readings to calculate the wavelength of the light where:
λ=
sw
D
Discussion
Compare your calculated wavelength to data about the wavelengths of visible light. Does the
colour of the beam match your calculated wavelength?
AQA Physics A AS Level © Nelson Thornes Ltd 2008
1
Topic 13.4
Double slit interference
Two source interference
Equipment and materials required
The experiment uses a ray box and needs to be carried out in a darkened room.
Equipment required:
 a 12 V ray box and a narrow slit
 a 12 V power supply
 cloth for moving the ray box if it becomes hot
 a millimetre rule
 a white paper screen attached to a wall or screen
 a metre ruler
 a double slit
A double slit can be made by printing a black rectangle with two white lines onto acetate.
The lines should be less than 0.5 mm apart. The slit separation can be measured by
photocopying the double slit, doubling its size each time until the separation is large enough
to be measured.
Alternatively, to make the double slit, paint a glass microscope slide with aquadag (or
similar) and scratch two parallel lines on it using a scalpel.
Safety
 Check all equipment for loose wires.
 Ensure furniture and bags are removed from walkways since the room will be dark during
the experiment.
 Tell the students to switch the ray box off if it becomes too hot.
 CLEAPSS information on ray boxes: Laboratory Handbook/CD-ROM section 12.16.
AQA Physics A AS Level © Nelson Thornes Ltd 2008
2
Topic 13.4
Double slit interference
Two source interference
Aims
 To observe a light beam split into two sources by a double slit.
 To observe the interference pattern created using double slits and a ray box and take
measurements in order to calculate the wavelength of the beam.
 To recognise the appearance of the interference fringes produced by a double slit system.
Safety
 Check all equipment for loose wires.
 Ensure furniture and bags are removed from walkways since the room will be dark during
the experiment.
 Tell the students to switch the ray box off if it becomes too hot.
 CLEAPSS information on ray boxes: Laboratory Handbook/CD-ROM section 12.16.
Teaching notes
6 This experiment should take about 15 minutes.
7 The slit in the ray box needs to be as narrow as possible to make the fringes distinct but
wide enough so that the beam covers the double slits.
8 A double slit can be made by printing a black rectangle with two white lines onto acetate.
The lines should be less than 0.5 mm apart.
9 The slit separation can be measured by photocopying the double slit, doubling its size
each time until the separation is large enough to be measured.
Answer to question
The interference fringes would disappear. A single slit diffraction pattern due to the
remaining slit would be observed.
Discussion
The calculated wavelength will be in the middle of the range of visible wavelengths. The
white colour of the beam does not match the spectrum colour of the calculated wavelength
because the light from a ray box covers all the wavelengths of the visible spectrum and
therefore contains all the colours of the spectrum.
Practical assessment opportunities
1 ISA preparation:
practical task – practice in making reliable measurements and carrying out calculations
AQA Physics A AS Level © Nelson Thornes Ltd 2008
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