Absorption spectrum of sodium

Demonstration

Using a spectrometer to observe the absorption lines formed when white light passes through sodium vapour.

Apparatus and Materials

• Lamp with fine filament (12 V, 24 W)
• Lamp holder (S.B.C.) on base
• Power supply, low voltage, variable
• Positive lenses (+7D), 2
• Lens holder, 2
• Retort stands, bosses and clamps, 2
• Spectrometer
• Prism (preferably high dispersion)
• Bunsen burner with salt for sodium flame

Health & Safety and Technical Notes

Read our standard health & safety guidance

It is essential to provide an intense sodium flame.

Place the flame midway between the two lenses where the first lens will produce a sharp image. This will make all the white light pass through the flame in the region where it is rich in sodium.

Focus the spectrometer in the usual way. Keep the slit narrow.

Adjust the voltage applied to the lamp as well as the slit width to get the best conditions for seeing the dark lines.

Procedure

1. Direct white light from the line filament lamp to the slit. Arrange the two positive lenses to make sure that all the white light entering the collimator of the spectroscope will pass through the flame on the way to it.
2. Dip an iron wire or a ceramic rod in concentrated brine and hold it in a Bunsen flame to provide the intense sodium flame, or use the filter-paper method. See technical note for this experiment:

   Spectra formed by gratings

Teaching Notes

• One lens forms a real image of the filament in the flame; the other lens forms a real image of that first real image on the slit of the spectroscope. It is easiest to make each of the distances, filament to lens, lens to image, image to lens and lens to slit, twice the focal length of the lens concerned.
• The spectrum from the white light passes through cooler sodium vapour. The cooler sodium vapour absorbs energy from the white light spectrum, producing dark lines.

This experiment was safety-tested in February 2007