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Practicals

Studying Spectra
AKA: Observing Spectrums

Summary

Students observe various spectra created by different light sources.

Operation

A spectrum describes the range of colours visible when light is split using a prism. This practical involves students using diffraction gratings to view several types of light source and observing the spectra they create.

Diffraction gratings are slide-like pieces of glass with fine lines etched on to one surface. When light travels through the pane it is diffracted through several different angles, effectively splitting the light up into its comprised wavelengths or colours.

The gratings can be held up to the eye and a light source viewed through it. Around the source you should be able to see several small spectrum patterns. Depending upon the type of light source used, you may see different colours within the spectrum.

Some sources may need slits (the dimensions usually used in rayboxes) placed in front of them in order to get a 'clean' spectrum rather than several blurry images.

Equipment typically required (per set)

  • diffraction grating (300-500 lines per mm)
  • green 'gel' filter
  • red 'gel' filter
  • access to the following light sources (as many as possible):
    • 12V standard lamp
    • hydrogen spectrum tube
    • neon spectrum tube
    • standard fluorescent tube
    • white LED
    • sodium flame
Students typically view the sources from a distance of several metres to get the best spectra. A sodium flame can be created by burning a stick of sodium chloride in a Bunsen flame.

Diffraction gratings can be easily broken if dropped. You can limit the risk by sticking a few layers of electrical tape to the edges which can soften the impact if dropped.

Gel filter are the types used in theatre lighting. Your drama department should be able to either supply you with some or a supplier from which to purchase some. Sheets are inexpensive and students only need pieces that are 10cm3 at most.

Safety

 CautionWhen using an EHT unit, ensure that all connections between the tube and holder are made before switching on. Also be aware that the tube holder should have no exposed metal which may become live during operation. Refer to professional guidance before using an EHT device.

The contents of this page are for information only. Please refer to CLEAPSS, SSERC or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.