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Grandpa Pencil
explores the secrets
of the
Doppler Effect

 


 


The Doppler Effect

Experiment with the Doppler Effect

You will need a friend in a car with a whistle.

  • Have a friend drive slowly past you blowing the whistle.
  • Listen for any change in the sound of the whistle.
  • There should be no variation.
  • Have your friend drive past quite quickly while blowing the whistle.
  • This time you will hear the pitch of the whistle rise as your friend approaches and fall as the car passes.

The sound of a steady frequency, will emit a number of complete sound waves (a series of push and pull changes in pressure.) in one second.

If the source is stationary relative to the listener, the ear receives that number of sound waves in a second.

If the source of sound is moving, the motion of the air in front is compressed by the vibrations more than x times a second and the air behind compressed less than x times a second.

Therefore an observer in front of the moving source will hear a higher frequency and behind, a lower one.

The Doppler Effect was first predicted by the Austrian physicist, C. J. Doppler in 1842.

Doppler was not able to experiment extensively because there were no vehicles capable of travelling fast enough at the time.

The first recorded demonstration of the Doppler Effect was carried out by C. H. D. Buys-Ballot, a Dutch scientist whose main work was in meteorology.

A trumpet was the sound source and was conveyed on a steam locomotive.

Light and sound are both forms of wave motion and the Doppler Effect has also been observed where a light source, such as a distant star, is moving.

The change in frequency of light is seen as a colour change in the light.

Light of high frequency is seen as blue while light of low frequency is seen as red.

A decrease in the frequency of light gives a change in colour from blue to green, from green to yellow, from yellow to orange and from orange to red.

A shift of the pattern of lines observed in the spectrum of a star is useful to scientists as it gives an indication of both the speed and direction of the star.


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