Doppler shift · moving source
Each crest leaves the source at the same speed c, but the source has already moved on before the next crest is emitted. Ahead of it the crests bunch up (higher pitch), behind it they stretch out (lower pitch). This is why an ambulance siren drops in pitch as it passes, and why the light of receding galaxies is redshifted.
right-click · learn & record
Sound speed in air c = 343 m/s. Source speed v and emitted frequency f set the pitch the listener hears as the source approaches (cos θ > 0) and recedes (cos θ < 0).
