Projection of spectrum with diffraction gratings
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
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
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.
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.
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.
a Place the coarse grating in the beam just beyond the lens.
b Place red and then green filters in the beam and show the effect of colour absorption by the filters.
c Finally, replace the coarse grating by a finer one and the spectrum will spread out more.
1 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).
3 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.
4 For an image of the fringes produced, see the web link below. Thanks to Jay Jamaican for suggesting that we add this link.
5 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