C.5.2—Doppler wavefront diagrams

Start with the reference case

A stationary point source emits circular wavefronts whose centres remain at the source and whose spacing is the emitted wavelength. Use this as the comparison before adding motion. The wavefronts travel through the medium at the wave speed.

Move the source

If the source moves toward a stationary observer, successive wavefronts are emitted from progressively advanced positions, so the wavefronts are compressed in front of the source and spread behind it. The observer in front receives a shorter wavelength and higher frequency; behind it, the wavelength is longer and the frequency lower.

Move the observer

If the source is stationary and the observer moves toward it, the wavefront spacing in the medium is unchanged. The observer meets wavefronts more often, so the observed frequency increases. Moving away gives a lower observed frequency; it does not change the wavelength in the medium.

Common trap

For a moving source, shift the centres of successive wavefronts; for a moving observer, keep the wavefronts equally spaced and change only the rate at which the observer encounters them. Do not combine moving-source and moving-observer cases unless the question explicitly asks for both.