Spectra formed by gratings

Demonstration

Observing the spectra from a variety of light sources.

Apparatus and Materials

For each student or group of students:

• Fine diffraction grating (about 300 lines/mm)
• Light source, compact
• Neon lamp
• Hydrogen discharge lamp
• Sodium lamp or Bunsen burner and salt
• Clear linear filament striplight in a mounting without diffuser, wired with 13 A plug
• Appropriate power supplies for lamps

Health & Safety and Technical Notes

The UV intensity from the compact light source will be reduced by the distance between the lamp and the grating. Do not allow anyone to view the spectrum from a distance below 3m.

Read our standard health & safety guidance

The sodium source can be a lamp or salt (stick or granular) in a Bunsen flame. Alternatively, you can make a sodium flame using a filter paper. The paper is soaked in brine, folded in half and wrapped around the mixing tube of the Bunsen burner with the fold just in the flame.

Procedure

Set up the light sources at one end of the laboratory and hold the grating close to the eye. Look at the distant lamps and examine the spectra seen.

Teaching Notes

• A white-hot filament will show a central white line where waves of all colours go straight through the grating. Out to each side is a bright band which corresponds to the first bright fringe out from the centre of Young's fringes (one wavelength path difference). Since the light is white, each bright fringe is spread into a wide spectrum of colours. Further out to each side, there may be a wider but fainter spectrum which corresponds to the next bright fringe out from the centre (two wavelengths path difference).

Image courtesy of Tony Reynolds
• The neon source is a tube containing neon atoms which are being bombarded by high speed electrons. The red light comes from neon atoms as they recover from that excitation. There will be other colours too.

Image courtesy of Tony Reynolds
• The hydrogen source is a tube containing hydrogen atoms which are being bombarded by high speed electrons at high voltage. As the atoms recover they give out only a few definite colours.
• The light from a sodium source is pure yellow, and not a mixture of red and green which we accept for yellow in colour mixing. Many commercial lamps also contain neon so the spectrum also contains weak red lines.

Image courtesy of Tony Reynolds
• The long filament strip light (about 25 cm) is a useful white light source because coloured filters, red, green, blue, can be wrapped around it. The displacement of the red, green or blue light will be clearly seen, showing that red light is diffracted the most.
• An interesting extension is to record the spectra photographically. The diffraction grating is taped to a camera lens and the camera focused on the light source (the camera is replacing the eye). With coloured film loaded into the camera, or a digital camera, then a few seconds exposure will produce wonderful spectra photographs.

This experiment was safety-tested in January 2007