Earth and Space


Practical Activity for 11-14 PRACTICAL PHYISCS

Using a group of students as light waves, it is easy to show how light can be red and blue shifted when astronomical objects are moving relative to one another.

Apparatus and Materials

  • Students
  • A stop watch
  • A camera to record the demonstration

Health & Safety and Technical Notes

Read our standard health & safety guidance

Teaching Notes

  • Red and blue shifting of light is a difficult idea to grasp without a visual aid. This demonstration allows you to use students in a practical way to demonstrate the idea of light changing wavelengths when the emitter is moving relative to the observer.
  • This demonstration really needs to be done outdoors or in a large school hall. You might have some limited success in a classroom.
  • The greatest difficulty in making this demonstration work is organising the students so they know what they are doing. Although the idea of walking backwards and forwards whilst waiting to be told you have been emitted as a wave of light sounds simple, it can be quite tricky to get right at first. However, it is worth the effort to see the result.
  • You may need to check that your time keeper is calling the times accurately, it is easy to get distracted by having to start a stop watch and call out “go” when a light wave needs to be emitted at the same time.
  • It is worth noting that we have made no distinction here between redshift caused by the Doppler effect and cosmological red shift caused by space expanding between different parts of the Universe.
  • Using a video or stills camera to record the results is very worthwhile so that a record can be kept of what was observed. It is also possible to mark the students on the images as peaks or troughs in the wave, and then draw completed waves on to the images so the difference in wavelength can be seen more clearly

can be described by the relation 1+z=Δλ/λ
is a special case of Doppler Shift
has the special case Cosmological Redshift
can be exhibited by Star
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