Hi,

It occurs because the frequency of the sound waves changes as the source of sound moves closer to or farther from the listener. The Doppler effect as several real-world applications. For example, besides police radar, the Doppler effects used by meteorologists to track storms.

Doppler effect is used to measure the speed at which stars and galaxies are approaching or receding from us, in a mechanism named red shift or blue shift.Redshift happens when light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to the red end of the spectrum.

Hope this helps.