The Doppler Effect


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 1.  The Doppler effect is named after an Austrian, Christian Doppler, who proposed it in 1842 in Prague. This is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source.
  • It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. The received frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.
  • In the animation, the pink circles are sound waves. When the car is moving to the left, each successive wave is emitted from a position further to the left than the previous wave.
    • So for an observer in front (left) of the car, each wave takes slightly less time to reach him than the previous wave. The waves “bunch together”, so the time between arrival of successive wave fronts is reduced, giving them a higher frequency.
    • For an observer in back (right) of the car, each wave takes a slightly longer time to reach him than the previous wave. The waves “stretch apart”, so the time between the arrival of successive wave-fronts is increased slightly, giving them a lower frequency. 1

2.  This wave pattern doesn’t just work this way with sound waves, but does the same thing for light waves

  • A radar detector that is used to catch speeders, does this by measuring the distance between waves bounced off the car – whether the vehicle is coming toward the police vehicle or moving away



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3.  Astronomers have used this same concept to measure the speed of galaxies in the universe

  • When an object a star or a whole galaxy) is moving away from us – the electromagnetic radiation (or light if you will) given off by it will have a longer associated wavelength. This causes the light to shift toward the RED end of the Color Spectrum.
  • If the object is coming closer to us, then the shorter associated wavelength will push the light toward the BLUE end of the Color Spectrum. 2

4.  Think of it like the waves around a duck as it paddles its way on a pond…

  • The waves in front of the duck are shorter…the waves behind it are longer 3


5.  In 1929, an American astronomer by the name of Edwin Hubble determined that the great majority of galaxies appear to be racing away from one another. In fact, the further they were from our own, the more rapidly they appeared to be moving.

  • More than this, he determined that it is not just the galaxies that are moving away from us, but space itself was expanding in every direction.
  • This is referred to as Hubble’s Law of Expansion 4