Fine cloth as a grating
Observing a two-dimensional diffraction pattern.
Apparatus and materials
Light source, compact
Retort stand, boss, and clamp
Power supply, low voltage, variable, to supply 8A at 12V
Woven cloth, many pieces
Health & Safety and Technical notes
Use a compact light source, rather than a line filament lamp, since this is a two-dimensional grating.
The cloth should be of a simple square weave with holes between the threads to let the light through. The samples should be large enough for students to hold with two hands and try stretching and shearing. Old umbrella cloth works well.
Each student looks towards the lamp and holds a piece of cloth in front of one eye. Ask them to observe the pattern as they try stretching and shearing the fabric.
The cloth acts like crossed gratings. Indeed, as preparation, it might help to show two gratings, initially in line, and then rotate one.
Encourage students to be observant outside the lab. Looking at sodium street lamps through domestic net curtains is very effective, especially if there are lamps at various distances.
This experiment was safety-checked in February 2006