CP2: Investigation of interference effects to include the Young’s slit
experiment and interference by a diffraction grating
AT a, e, j Objectives
To determine the wavelength
CPAC - Teacher comment Evidence Done
of laser light using Young´s
Correctly follows written instructions to fringes.
1 carry out experimental techniques or Equipment
procedures.
Selects appropriate equipment and
2d
measurement strategies in order to ensure
Safety
suitably accurate results.
Complete Risk Assessment
Identifies hazards and assesses risks
associated with these hazards, making
3a safety adjustments as necessary, when
carrying out experimental techniques and
procedures in the lab or field.
Uses appropriate safety equipment and
3b approaches to minimise risks with minimal
prompting.
Preparation - Introduction
1. Watch this video: https://www.youtube.com/watch?v=Iuv6hY6zsd0
2. Find out what is meant by coherent light. Why does the light have to be coherent in this experiment? Laser
light is coherent – can coherent light be obtained without a laser?
3. How would the experiment change if you used light of a different wavelength?
4. Risk assessment: indicate all risks, and how you will minimize them
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Part A: Young´s double slit interference experiment
1. Set up laser as shown (D should be aprox. 1m)
2. Move the laser so that the light shines through the double slit slide. An interference pattern
should be visible on the screen.
3. Measure the fringe spacing, w, over a large number of visible fringes. (Measure across several
fringes and divide this measurement by the number of fringes.)
4. Measure the distance, D.
5. Repeat for a number of different distances, D.
6. Make note of the slit separation, s. This is printed on the slide.
7. Draw a graph of w against D.
8. Compare your calculated value with the manufacturers value.
Part B: Diffraction grating
1. Place the laser approximately 4m away from a large wall and place the diffraction grating in front
of the laser. The laser beam should pass through the grating at normal incidence and meet the
wall perpendicularly.
2. Measure the distance, D, between the grating and the wall.
3. Turn on the laser and identify the zero-order maximum (straight through)
4. Measure the distance x from the zero-order maxima to the first order maxima (closest to the
zero-order maxima on either side). Calculate the mean of these two values. This is the maximum
produced according to n = 1 using the equation nλ = dsinθ.
5. Measure the distance x for increasing orders (n=2, 3, 4…)
6. Use your values of x and D to determine θ using tanθ = x/D
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Your Write-up must include
o Cover sheet with CPAC Criteria
o Introduction / Theory
o Risk Assessment (what are the risks and how will you minimize them?)
o Procedure (method, variables etc. )
o Results (results tables)
o Analysis of results – draw a graph of w against D.
o Analysis of results – find the value of wavelength for both investigations
o Calculations – (include any calculations that you have done)
o Conclusion/Evaluation – (discuss what you found, compare to the manufactures value)
Questions to consider in your report:
1. Explain why your chosen ruler would is suitable for measuring the distances in this experiment.
2. Describe the diffraction pattern that would be produced if a narrow bright light source was used
instead of the laser.
3. Explain how the interference would be different if a green laser was used instead of a red one.
4. In Part B, you estimated the uncertainty in your value for the wavelength of the laser light. What
could have been done in this experiment to reduce this uncertainty?
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