The Doppler effect is the shift in frequency of a wave that occurs when the wave source, or the detector of the wave, is moving. Applications of the Doppler effect range from medical tests using ultrasound to radar detectors and astronomy (with electromagnetic waves).

We've all observed the Doppler effect with our ears. A good example is when an emergency vehicle passes with its siren going. When the vehicle comes toward you the siren frequency is higher than usual. The frequency shifts abruptly to lower than usual when the vehicle passes you and moves away. A similar effect occurs if the sound source is stationary and you move toward it or away from it.