Learning Aims:
  • Understanding that light can be diffracted by small objects
  • Understanding that examining the diffraction pattern can tell us the dimension of those objects
  • Understanding that the diffraction pattern depends on wavelength
    • CD, DVD and/or Blu-ray discs, Red laser pointer, Retort stand, Graph paper ‘screen’, adjustable slits
    Suggestions for use:

    The students should be asked to consider the differences between a CD and DVD/Blu-ray disc with the discussion being guided towards the amount of information each disc can store (CD: 740 MB, DVD: 4.7 GB, Blu-ray: 25 GB for single sided discs). They should then be asked to compare the physical size of each disc (120 mm diameter x 1.2 mm thick) and discuss why, if the discs are the same physical dimensions, the information content is different.

    Hand out Worksheet 2.5.

    The students then set up their laser so it is incident at some angle on the CD surface and should observe and sketch the diffraction pattern observed on the screen. If the students have not previously covered the topic of diffraction, they are likely to explain this pattern in terms of reflections from multiple objects. This observation will conform to their knowledge of CDs having ‘pits’ on the surface. They then replace the CD with a DVD and should note that the pattern is now broader.

    These observations should now be related to the amount of information stored on each disc. The conceptual difficulty students may encounter is that the pattern obtained from a DVD seems broader, which they may attribute to larger features on the DVD. However, the storage capacity of the DVD is larger than a CD, which suggests the features should be smaller!

    The students should then investigate how the diffraction pattern changes as laser light is passed through a slit that is made progressively narrower. This serves to reconcile the broad pattern obtained from the DVD and the smaller features on that disc.

    The exercise can be used as an introduction to diffraction, with the teacher providing the subsequent theory.

    Possible questions:
    • What does the size of the pits mean in relation to the amount of information that can be stored on the discs?
    • Why are red lasers used to read CDs but blue lasers are needed for Blu-ray discs?
    • Why do optical microscopes have a limit on their magnification?