Projection of spectrum with diffraction gratings

Demonstration

Exploring the diffraction pattern produced from white light.

Apparatus and Materials

• Coarse diffraction grating (about 100 lines/mm)
• Fine diffraction grating (about 300 lines/mm)
• Cylindrical convex lens
• Green Filter
• Red Filter
• Screen, white
• Lens holder, 2
• Light source, compact
• Power supply, low voltage, variable, to supply 8A at 12V
• Retort stands, bosses and clamps, 3

Health & Safety and Technical Notes

Read our standard health & safety guidance

The diffraction gratings should not be blazed.

If you don’t have a compact light source (quartz iodine lamp), a 48W 12V lamp will probably be bright enough to project the spectrum across the laboratory. The lens will produce plane waves.

The screen should be a long one, perhaps a 3 to 5 metre roll of white kitchen paper. A white wall is good.

Alternative arrangement

A slide projector can conveniently be used, instead of the compact light source, as illustrated opposite. In this arrangement a single slit must be inserted in the projector as shown.

Procedure

1. Set up the lamp and lens at one end of the darkened laboratory. Obtain a sharp image of the filament on a screen at the other end.
2. Place the coarse grating in the beam just beyond the lens.
3. Place red and then green filters in the beam and show the effect of colour absorption by the filters.
4. Finally, replace the coarse grating by a finer one and the spectrum will spread out more.

Teaching Notes

• You many want to let students use a coarse grating for themselves, before they are shown this demonstration.
• Talk students through the observation. Students look at the white-hot filament of a lamp with a grating held close to the eye. They should see a central white line where waves of all colours go straight through the grating. Out to each side, they should see a bright band, where light arrives from adjacent slits with one wavelength path difference. Since the light is white, each bright fringe is spread into a wide spectrum.
• Looking further out to each side, they may see a still wider, but fainter spectrum which corresponds to the next bright fringe out from the centre (two wavelengths’ path difference).
• Help students to make the link between colour and wavelength. Finer gratings will spread the light more.
• For a diffraction grating d sin A = n (wavelength), where A = angle at which the light appears, n is the diffraction order (1,2, ...), = wavelength, d = spacing between slits.
• For an image of the fringes produced, see the web link below. Thanks to Jay Jamaican for suggesting that we add this link.
• ALTERNATIVE: This video, from the National STEM Centre eLibrary, shows how to produce a diffraction pattern using a laser source and a thin, straight wire.

This experiment was safety-checked in February 2006