3.18 The Doppler effect

Wavecrests spread out from a source of waves. For the observer toward whom the source is moving, the wavecrests are closer together than they would be if the source were at rest. The observer sees a shorter wavelength and higher frequency. For the observer away from whom the source is moving, the wavecrests arrive farther apart. This observer sees a longer wavelength and lower frequency. It doesn't matter whether the observer or source is moving. Only the relative motion of the two is important.