Page "Doppler effect" Paragraph 1

An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding.

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 wavefront s 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.