Doppler effect is a change in frequency of a wave whenever there is a relative motion of the source and the observer..Frequency increases(Pitch becomes higher) when a source is approaching an observer. Frequency decreases(Pitch becomes lower) when a source has passed the observer...
Example scenario:..You are standing on the side walk when an ambulance passes by...As it approaches and nearing you, you noticed that the siren wails at a faster rate(increasing frequency) and at a higher pitch(noisy). And when the ambulance is right in front of you and it then passes you, you noticed that the siren wails a little bit at a slower rate compared to as the ambulance is approaching,and the pitch becomes lower(somewhat like flat sound).....
The scenario above is the example of Doppler Effect...and the most significant application of this Effect is in Radar technology specifically on Speed Detection Radars for overspeeding vehicles..
