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Light intensity variation across a diffraction pattern

IntroductionEducational data logger

This experiment investigates the variation in light intensity across a monochromatic diffraction pattern produced by a diffraction grating.

Equipment required

Experiment setup

The experiment was set up as shown in the diagram below.

Educational data logger

The laser pointing and grating were fixed in position (using Lego blocks) and the diffraction pattern displayed on a white card placed about 5 cm beyond the grating. The card was removed and the DrDAQ data logger put in its place. The height of the laser was then adjusted so that the diffraction pattern (a series of dots) coincided with the internal light sensor.

Either PicoLog or PicoScope software could be used for this experiment, in this case PicoLog was chosen. PicoLog was set up to take samples from the light sensor every 100 ms for 1 minute.

Carrying out the experiment

PicoLog was set to start recording data and to display a graph of the results (axes were set to automatic).

Once recording had started DrDAQ was moved slowly across the diffraction pattern at right angles to the optical axis. A pattern of maxima and minima should start to appear on the graph.

Questions and discussion of results

  • Explain why the pattern occurs
  • Why did the minimum light level not drop to zero?
  • How would the pattern be affected if the DrDAQ was further away from the diffraction grating?
  • Why are not all the maxima the same height?
  • Calculate angular positions of the maxima to m=3. What is the maximum order? (Assume: Wavelength = 670 mm, diffraction grating has 3000 lines cm-1)

Further study

  • How does the speed of movement of DrDAQ affect the results?
  • Try putting a small open ended paper tube on the light sensor to reduce the pickup of background light.
  • Try placing a red filter and/or a 1 mm slit in front of the light sensor to minimise ambient light and improve resolution.