Have you ever listened to the sound of a fire engine as it rushes past you with siren blaring, or to a train zipping by while tooting its whistle? The pitch of the sound drops as the source speeds past you. Is the pitch really changing? If you were on the fire engine or the train you would hear no change. This is called the Doppler Effect, named after Christian Doppler who explored and explained the phenomenon in the 1840s.

Do you want to know more about Christian Doppler and his experiments with sound? Here we see Christian at a railroad station listening to the sound of violin music coming from the train. Click the image below to learn more about Doppler's experiments and hear the Doppler Effect in action.