Doppler Effect

Acoustics · 1842

Equation: f′ = f·(c + vₒ)/(c − vₛ)
Field: Acoustics
Named After: Christian Doppler
Symbols: f emitted frequency · f′ observed frequency · c wave speed · vₒ observer speed toward the source · vₛ source speed toward the observer
Year: 1842

Significance

Motion squeezes waves ahead and stretches them behind — the siren that drops in pitch as it passes. The same effect in light is how a radar gun catches a speeder, how a star's wobble betrays an orbiting planet, and how Slipher and Hubble first read the galaxy redshifts, as recession velocities. That last reading is the useful approximation rather than the mechanism: a cosmological redshift is space expanding underneath the light while it travels, not motion through space.