The acoustic Doppler effect can be experienced day in the real world every: An ambulance siren sounds sharper closer to the emergency vehicle; when the vehicle pulls away, the siren sound descends. This is due to the change of wavelength of the sound waves, which are compressed or stretched during the movement of the sound source, thus changing its pitch. The effect applies to all kinds of waves, including light waves. Similarly, as a star moves away from Earth, its emitted light wave is stretched, creating the so-called red-shift, i.e. a longer wavelength of light.
Conversely, a light wave emitted from a star approaching Earth will be compressed, which causes a blue shift. In 1842, the Austrian physicist Christian Andreas Doppler predicted this optical effect in his paper “On the colored light of the double stars and certain other stars of heaven,” and presented this phenomenon to the Royal Bohemian Society of Sciences in Prague. Three years later, the Dutch physicist Christoph H. D. Buys-Ballot observed the acoustic Doppler effect in a spectacular experiment. He used the fastest transportation tool at that time—the railway. A musician was playing trumpet on a moving rail car while musicians standing next to the track listened to the tones he played. The displacement of the pitch in the tones they heard as the train passed by is equivalent to the predictions for Doppler’s color shift of light.
Today, the Doppler effect has made possible a number of technological achievements in fields such as speed measurement via traffic cameras, GPS, and the ultrasound measurement of blood flow velocity in the human body. In addition, the Doppler effect plays a key role in such important quantum phenomena as the broadening of spectral lines and the trapping and cooling of atoms with laser light.