Spectra

Class practical

Students use a diffraction grating as a tool for observing the spectra from a variety of light sources.

Apparatus and Materials

• Fine diffraction grating (about 300 lines/mm)
• Green filter
• Red filter
• L.T. variable voltage supply (capable of 8 A at 12 V)
• Lamp, 12 V 24 W
• Lamp holder on base
• Spectrum tube, hydrogen
• Spectrum tube, neon
• Spectrum tube holder, with integral power supply or 5 kV EHT power supply
• Sodium flame

Health & Safety and Technical Notes

Where the EHT supply is used, all connections between the tube holder and the supply must be made before the supply is switched on. The tube holder should not have any exposed metal which could become live.

Read our standard health & safety guidance

Each student pair will need a fine diffraction grating, red filter and green filter.

The light sources should be mounted as high up in the laboratory as possible. The spectrum tubes will require an appropriate holder and voltage supply.

Procedure

1. Set up the bright, white-hot filament high at the end of the laboratory.
2. Ask students to observe the light source with the fine grating held close to the eye.
3. Students should then look at the neon spectrum tube; at the hydrogen tube; at a slit held in front of a sodium flame; at the bright, white-hot filament, through red and green filters.

Teaching Notes

• The capillary-tube gas-filled lamps which operate on (3-5) kV are in fact a fine-line source of light; neon tubes produce spectra consisting of many bright lines of different colours, whereas hydrogen is much fainter and produces fewer spectral lines.
• Sodium lamps normally need to have a slit placed in front of them in order to produce a line source. Sodium light can also be produced by holding a sodium chloride stick in a Bunsen burner flame, or even by sprinkling sodium chloride into the flame, though that can be quite messy.
• Alternatively, use the technique in this experiment:

  Interference with air wedge

• When a tube containing neon gas, for example, 'is connected to a high voltage supply, it produces a 'spectrum because electrons bombard atoms and excite them.' ''The tube gives out red light 'that comes from neon atoms as they recover from that excitation.'' Because many electron transitions are possible, neon produces many other colours too.'' The hydrogen atom is much simpler than neon so there are fewer spectral lines.

This experiment was safety-tested in February 2006