The Doppler Effect

 Created on Wednesday 22 Oct 2008
by Σπύρος Κιτσινέλης
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What could be the relationship between

  • a speeding ticket
  • Alpha Centauri
  • the sound of an approaching ambulance fighting through traffic
  • our national security
  • a blood circulation diagnosis?

The relationship all the above have is that they are the result of the use of the same phenomenon. The importance of this phenomenon is illustrated by the multiple applications in everyday life so it is worth explaining it. The name of it is the Doppler Effect, named after an Austrian physicist called Christian Doppler. Mr. Doppler in 1842 at the age of 39 published a paper that gained him immortality. He realized that a wave, whether it is an electromagnetic wave traveling even in vacuum or a wave needing a medium such as sound through air, changes its wavelength and therefore its frequency if the source of the wav is moving in relation to an observer.Τα ηχητικά κύματα της σειερήνας του περιπολικούLet us use an example from everyday life in order to understand what Mr Doppler described. And it certainly was something clever this man said, judging by the frequent use of this effect in so many aspects of life.Let us say for example that I begin to throw tennis balls at a steady rate towards a wall at a certain distance away from me. Let us say that I throw one ball per second. The wall therefore will receive a ball per second. If however, without changing the rate at which I throw the ball, I start walking towards the wall, then I still throw one ball per second but the wall is receiving them at an increasing rate. The opposite will happen if I start walking backwards away from the wall.

Essentially that is all. We can estimate whether a star billions of kilometers away, is moving towards or away from us by looking at its emitted light and checking whether its electromagnetic waves are increasing or decreasing in wavelength or as the astronomers call it, whether the light is blue-shifted or red-shifted.

By sending radiowaves towards a car on the national road we can see its speed by checking the amount by which the reflected radiowaves have decreased in wavelength. The same radiowaves if sent up to the skies will bounce off an incoming enemy aircraft revealing its presence and of course it speed and direction via the same Doppler effect.

We can also direct sound waves or even shine laser light towards the arteries of a patient and check the normality of his blood flow. I am sure some might have already seen the word Doppler in various clinics.

And finally if you are stuck in traffic and you hear the siren of an ambulance, its pitch will tell you whether the ambulance is approaching or moving away from you. If the pitch is getting higher then make some space because it is approaching. And in case you are not stuck in traffic…well then don’t press the accelerator too much because like we said Doppler and radars are working for the police.