Fine cloth as a grating

Class Practical

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

Read our standard health & safety guidance

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.

Procedure

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.

Teaching Notes

• 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-tested in February 2006