Doppler Effect

You know the sound of a car passing by changes. Not only the intensity (loudness), but also the frequency (wavelength).

• As the source comes towards the observer larger frequency (shorter wavelength) is heard by the observer


• As the source moves away from the observer smaller frequency (longer wavelength) is heard by the observer

Try the applet and take a look to separation of the wavefronts (wavelength).
What happens if the source is moving at the spped of sound or even at a supersonic speed?

The effect takes place if the observer moves also. Or both move. As long as they are in relative motion.