In this diagram, the source is at rest. Waves are emitted in both directions at the same rate, and the separation of the wavefronts is one wavelength.
In this diagram, the source is moving to the right. As when the source is at rest, waves are emitted in both directions at the same rate, as you can see by watching the source carefully. Compare the moving wavefronts with those in the above diagram to convince yourself that all the wavefronts have the same speed. The separation of the wavefronts in the forward direction (to the right) is less than one wavelength, because the source is moving in the same direction that these wavefronts are travelling. The separation of wavefronts in the backward direction (to the left) is greater than one wavelength.
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