Interference
Quantum and Nuclear | Light, Sound and Waves

Diffraction demonstrations

Classroom Activity for 14-16 Supporting Physics Teaching

What the Activity is for

The purpose here is to show diffraction, to demonstrate that the light spreads. To do this you will need to practice and take care – the setup is not straightforward. However, it is not a situation where the physical can be replaced by images or by computer simulations. The sense of awe, wonder and disquiet engendered by physical immediacy is important.

What to Prepare

  • a bright and focused light source
  • a translucent screen
  • a dark plate with a 1 mm pinhole in the centre
  • a darkened laboratory
  • several objects as obstructions to be placed in the narrow beam (we suggest a dark sheet with holes of 2 millimetre, 3 millimetre and 1 cm in diameter; human hair; a ball bearing 3 millimetre in diameter; a round-headed pain; a set of three slits 3 mm, 0.3 mm and 0.03 mm across)

What Happens During this Activity

Place the lamp and pinhole at one end of the laboratory to throw a sharply defined beam of light across the room. Place the objects 3 metre from the lamp and then place the screen a further 2 m from the object. The students look at the screen towards the lamp: they stand behind the screen.

Introduce the objects to the students one at a time. Draw out expectations for the shadow that will be cast. Place the objects one at a time into the beam. To do this you'll need to mount the object to make it easy to hold, say on a piece of plate glass or perspex – whatever it is it needs to be transparent. The idea here is to replace the simple rule that light travels in straight lines, so producing sharp shadows, with a more subtle interpretation that draws on the idea of this topic. The patterns are truly awesome. They are one of the strongest pieces of evidence that students will see to suggest that one needs a proper model of radiations and radiating to understand the phenomena. Here they have seen all the examples, but you might well draw connections to hearing things, where sound often travels around corners. This is less surprising, so perhaps not such a good place to start, but the connections between the behaviours of light and sound need to be made.

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