Doppler Effect
Motion of the source or observer compresses the wavefronts ahead and stretches them behind, raising the frequency you receive as it approaches and lowering it as it recedes.
Doppler Effect
The Doppler effect is the change in observed frequency caused by relative motion between a Wave source and an observer. It is the rising-then-falling wail of a passing siren, the pitch drop of a race car, and — for light — the redshift that tells us distant galaxies are flying away. The wave's speed through the medium does not change; what changes is how the wavefronts pile up.
Why the fronts bunch
Picture a source emitting a crest every period T. If the source is moving toward you, it creeps a little closer between successive emissions, so each new crest leaves from slightly nearer than the last. The crests crowd together ahead of the source — shorter wavelength, higher frequency. Behind the source they spread apart — longer wavelength, lower frequency. The wavefronts themselves stay perfect circles; it is only their centers that march forward.
For a source moving at speed v_s through a medium where waves travel at v, the frequency an observer receives is
with the minus sign (higher pitch) when the source approaches and the plus sign (lower pitch) when it recedes.
The supersonic limit
Push the source past the wave speed and the geometry changes character. The source now outruns its own wavefronts, which stack up along a cone — the Mach cone. For sound that cone is the shock wave heard as a sonic boom; for a charged particle in a medium it is the optical analogue, Cherenkov radiation. The Doppler formula above diverges at v_s=v precisely because the fronts ahead collapse onto a single surface.