What Is an Example of the Doppler Effect?

What Is an Example of the Doppler Effect?

An example that is often given to illustrate the Doppler effect is the rising and falling tone of a train. The effect is most obvious if the train blows its horn as it approaches, and then passes by a stationary observer next to the tracks.

As the train approaches, the horn generates compression waves that arrive closer and closer together, as heard by the track-side observer. As the train recedes into the distance, the compression waves stretch out, producing a gradually falling tone. The horn's normal tone is only heard as the train passes directly in front of the observer.