In 1842 Austrian scientist Christian Doppler, discovered that when a source of sound and a listener are in motion relative to each other, the frequency of the sound heard by the listener is not the same as the source frequency. This effect works not only for sound but also for light and radio waves. This is the Doppler Effect, which has important applications in medicine and technology.

So far, we have only considered stationary sources of sound and stationary listeners (or observers). In this case, the frequency of sound reaching a listener’s ear will be the same as the frequency of the source when the source of sound, the medium and the listener (observer) are all at rest. If however, the source and the observer are in relative motion, the frequency of the wave noted by the observer appears to be different from the true frequency.

Key terms

• Doppler’s Effect: The variation in the pitch of a sound due to the relative motion between a source of sound and a listener.