Doppler Effect

Observe frequency shifts as source and observer move relative to each other

The Doppler effect occurs because a moving source continuously changes its position relative to the wavefronts it emits. As the source approaches, wavefronts pile up — wavelength decreases, frequency increases. As it recedes, wavefronts spread — frequency decreases. The Doppler formula applies to any wave: sound, light (redshift/blueshift), and even radar speed guns. When source speed equals sound speed (Mach 1), wavefronts accumulate at one point forming a shock wave — the sonic boom. At Mach > 1, the source outruns its own wavefronts, creating a Mach cone. In medicine, Doppler ultrasound uses these frequency shifts to measure blood flow velocity.