Light intensity variation across a diffraction pattern: results

The results obtained from the experiment are shown below.

Answers to questions

Q. Explain why the pattern occurs.
A. The multiple slits of the grating act as discreet light sources which interface destructively and constructively.

Q. Why did the minimum light level not drop to zero?
A. This was due to the background ambient light level.

Q. How would the pattern be affected if the DrDAQ was further away from the diffraction grating?
A. The maxima would be further apart but at the same amplitude as before.

Q. Why are not all the maxima the same height?
A. The envelope of the maxima follows the diffraction pattern due to a single slit since each slit produces this envelope.

Q. Calculate angular positions of the maxima to m=3. What is the maximum order? (Assume: Wavelength = 670 nm, diffraction grating has 3000 lines cm^-1)
A.  Θ₁=11.5 °,  Θ₂=23.5 °, Θ₃=36.9 °, m=4

Teachers’ notes

The maxima envelope shown above is not quite the expected theoretical one (there should be symmetry about the central axis with the 4th peak being the highest). This was probably due to the difficulty of keeping DrDAQ vertical during the scan. Mounting the board on a trolley, or using an external light sensor may have given better results.

Credits, comments and further Info

This experiment was performed and written up by Dr Cooke of Hallam Consultants.

Hallam consultants is a Sheffield (UK) based educational and scientific consultancy specialising in Physics. Bob Cooke can be contacted on 0114 2301283.