The Doppler Effect

A source emits sound waves as expanding spheres of compression and rarefaction, each new wavefront starting from wherever the source currently is. If the source moves, it chases its own outgoing waves. Ahead of it, successive wavefronts get crowded together, so the wavelength shrinks and the listener there hears a higher frequency. Behind it, the source is pulling away from the waves it already sent, so they stretch out, giving a longer wavelength and a lower frequency. The source's own vibration frequency never changes; only the spacing of the waves in space changes.

Moving the listener instead of the source gives a similar shift but for a different reason: the listener's own motion changes how fast they sweep through wavefronts, without altering the wavelength in the air at all. A listener moving toward a stationary source meets each crest sooner than if they stood still, so they detect a higher rate of arrivals. The two cases are not identical in formula, since the wave medium (air) is the fixed reference, but at everyday speeds they produce very similar shifts.

The same idea applies to light. Galaxies moving away from us stretch their light to longer, redder wavelengths, called redshift. Because almost every distant galaxy shows redshift, and more distant ones show more of it, the universe must be expanding, with space itself carrying galaxies apart.