Today we celebrate the birthday of Christian Doppler (29 Nov 1803 – 17 Mar 1853), the Austrian physicist who first described how the observed frequency of sound and light waves are affected by the movement of the source of the waves relative to the observer. The phenomena became known as the Doppler effect.

Simply put, sound and light waves would have a higher perceived frequency if the source was moving toward the observer and a lower perceived freqency if the source was moving away from the observer.

It is said that Doppler first tested his hypothesis by using two groups of trumpeters – one group stationary on a train station, and the other group on an open train car. Instructing them to all play the same single note, he found that, as the open car passed the station, the pitch of the two groups did not match. Approaching the station the trumpeters on the train appeared to play a higher note, and leaving the station they appeared to play a lower note.

One of the places where the Doppler effect is very obvious is at a motor racing event – I am sure everyone has heard (either live or on TV) the effect of the sound of a racing car, or motorbike, changing quite dramatically as it comes screaming past. As the car races forward, the sound waves emanating from the engine effectively gets compressed in front of the car, resulting in a higher pitched sound, while they get spread out behind the car, producing a lower pitch.

Because the extent to which the frequency changes is dependent on the relative velocity of the source, observed changes in frequency can be used to calculate the speed at which the source is traveling.

The Doppler effect finds application in a wide range of fields, from astronomy to radar to medical imaging to flow measurement to satellite communication and more.

As mentioned before the effect does not apply to sound only – it applies to all waveforms, including light. A light source moving towards the observer will appear to have a higher frequency than one moving away from the observer. However, because very high speeds are required to achieve an effect visible to the human eye, this is less easy to observe than the sound example.

There’s a classic physics joke that says the most effective way to observe the optical Doppler effect is to look at cars at night – coming towards you, their lights are all white, while moving away from you, their lights are red!  (Think about it, it makes perfect sense…) 🙂