What is the Doppler effect?

The phenomenon that explains the apparent change in wavelength of radiation (either sound waves or light waves) due to relative motion between the source and the receiver. This was first described, and hence named after, the Austrian physicist Christian Doppler in 1842. In simpler terms…

Imagine the source as a loud train whistle and the receiver as your ear. If the source is approaching the receiver, the frequency of the sound waves increases (and the wavelength decreases). The result would be an increase in pitch of the whistle. In fact, you could experience this first-hand if you listened as a train approached with its whistle blowing, then compare the sound to the lower pitch as it speeds away from you. The sound you’re hearing is being Doppler-shifted.

The same principle applies to light waves. A source approaching the receiver will likewise produce higher frequency waves. You may have heard the term “blue shifted” to describe this. Red shifted light is the converse, when the source and receiver are receding.

Source: The Handy Science Answer Book.