See how a moving source compresses and stretches wavefronts
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When a wave source moves toward you, wavefronts are compressed ā higher frequency (blue-shift). Moving away ā stretched ā lower frequency (red-shift). Formula: f = fā Ć v/(v Ā± vā). When vā = v (Mach 1), wavefronts pile up into a shock wave (sonic boom). This applies to sound, light (used to measure stellar velocities), and radar (speed guns).