Doppler Effect

The "Doppler Effect" happens when a wave's source is moving in relation to us: So a passing siren sounds like "nee-nee-nee-nee ... woooo-woooo", or a passing race-car sounds like "eeee-yoooo":
doppler approaching Approaching:  higher frequency
Leaving: lower frequency doppler leaving

This applies to all waves, including light waves and even waves on the sea:

Press Play!

About the Model

The source emits waves at a fixed frequency, but the observer receives a higher frequency when the source moves towards them (and lower frequency when the source moves away)

To make the model "honest" I based the received frequency purely on when a wave crosses the center of the blue circle.

Why does the sound change in little steps?

Because our model is not perfect! Each new wave, as it arrives, changes the average up or down. We could improve the model by using more data points for a smoother result.

Relative motion!

The model shows the source moving, but it is really the relative motion between source and observer that is important. Imagine you are in a car passing by a sound source ... you would also notice this effect.


The same model could also show light getting red-shifted for objects moving further apart, or blue-shifted when getting closer. Note that the source's color does not change, but the observation of its color does