Doppler-effect

 The Doppler Effect is something which occurs when something which emits sound or light moves relative to an observer. The object, observer, or both can move, causing an apparent change in the frequency of the wavelengths being emitted by the object. The Doppler Effect explains why a rude driver's car horn appears to change in frequency as he or she zooms by while leaning on it, and an understanding of the Doppler Effect can help scientists make a variety of observations about the world around them.