Doppler Effect

The classic example is the change in tone of the noise of a vehicle engine as it approaches, passes and leaves the observer. As the vehicle approaches, more sound waves per second enter the ears of the observer. The sound appears to be higher pitched that if the vehicle and observer were stationary. This is because more sound waves per second = a higher frequency and higher frequency sound is heard as a higher pitch.

As the vehicle passes the observer and moves away, the engine tone is heard to drop. This is because slightly fewer waves per second enter the ear. Thus, the tone is heard to fall (lower frequency).

The Doppler effect was once used by the emergency services to help them to get through traffic. The first ‘sirens’ were bells, then we had the ‘me-ma’, a two fixed tone siren. Both of these were subject to the Doppler effect and drivers could recognise from which direction an emergency vehicle was approaching. Then some bright spark (read: complete and utter plonker) decided that American sirens were somehow better. These have a continuously changing tone so it is now impossible to tell the direction the emergency vehicle is approaching until it is really close. A brilliant way to make it more difficult to get through traffic and a perfect example of the detrimental way that adopting aspects of the US culture affects us here in the UK. It is as bad as those people who reply ‘I’m good’ when asked the question ‘How are you’ which grammatically makes no sense whatsoever.

Anyway, back to the Doppler effect in connection with astronomy. The same happens to all waves, including light waves and radio waves. In the case of light waves, higher frequencies are bluer and lower frequencies are redder. An object with a negative radial velocity (moving towards the Solar System) will be blue shifted and vice-versa will be red shifted. The amount of red-shift is used to determine the distance of distant galaxies. The Doppler effect causes emission and absorption lines of a spectrum to be shifted from their normal position. The faster an object approaches or receded, the further the lines will be displaced.

 

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