It happens everyday – you hear the wail of a siren from a police car or fire truck. Even if you cannot see the vehicle, you know when it passes because of the change in sound. This is also true if you are holding at a railroad crossing and a freight train passes, blowing its horn. There is a distinct change in the sound of the horn…. Why is that?
THE CHANGE YOU HEAR IS CALLED THE DOPPLER EFFECT. (Click on the green car above to see animation.)
The siren on that police car is emitting sound waves at a particular frequency. When the car is speeding toward you, each of the sound waves is emitted from a position closer to you than the previous wave. Therefore each wave takes slightly less time to reach you than the previous wave. (In other words, the waves get to you faster - we call this an "increase in frequency".) The reverse is also true - if the police car is moving away from you, each wave is emitted from a position farther from you than the previous wave, so the arrival time between successive waves is increased, and we hear a lower frequency sound.
The Doppler effect also works on light waves – in fact scientists are able to measure changes in sound and light frequency to better understand the speed at which objects are moving. Radar guns used by police use the Doppler effect to calculate the speed of oncoming cars.
Think like a scientist and try to be aware of the Doppler effect the next time a police car, train, motorcycle, or airplane passes.