If a stationary source of radiation is observed by someone moving towards the source they will detect a crest in the wave more rapidly than the source produces them, i.e. the observed frequency will be higher than the source frequency. If the observer is moving away then the frequency will be lower. The same effect will be noticed if the source is moving and the observer is stationary. This is the Doppler Effect. The effect works for any wave type including sound and light. It is particularly important in astronomy as it allows us to calculate the radial velocity of stars, gas clouds and whole galaxies, giving us a means of determining the dynamics of the Universe.
The following simulation program allows students to practice using the Doppler Effect and to extract information about the motion of astronomical bodies.
The Doppler Effect is probably one of the most versatile tools ever utilised in astronomy. In addition it underlies probably the most important astronomical discovery of the 20th century, the Hubble expansion of the Universe.
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