Doppler Shift Calculator (Sound & Light)

Doppler Effect Guide

What the Doppler Effect Is

The Doppler effect is the change in frequency (and wavelength) of a wave when the source and observer move relative to each other. The classic example: an ambulance siren sounds higher-pitched as it approaches, then drops to a lower pitch as it passes and recedes. As it approaches: the sound waves are 'compressed' (shorter wavelength, higher frequency, higher pitch). As it recedes: the waves are 'stretched' (longer wavelength, lower frequency, lower pitch). It applies to all waves: sound, light,

Sound Doppler

For sound, the medium (air) matters — the speed of sound is fixed relative to the air. Formula (source moving toward stationary observer): f_observed = f_source × v / (v - v_source). Where v = speed of sound (~343 m/s in air at 20°C). v_source = speed of the source. Source receding: use (v + v_source) in the denominator (frequency drops). Observer moving: slightly different formula (the observed speed of the waves changes). Examples: ambulance siren at 700 Hz, travelling at 30 m/s toward you: f

Light Doppler and Redshift

For light, there's no medium — the speed of light is constant for all observers (special relativity). The relativistic Doppler formula applies. Redshift: when a light source moves away, its light shifts to longer (redder) wavelengths. Blueshift: when approaching, light shifts to shorter (bluer) wavelengths. Redshift parameter z: z = (λ_observed - λ_emitted) / λ_emitted. For small velocities: z ≈ v/c (velocity / speed of light). Cosmological redshift: distant galaxies are redshifted because the u

Real-World Applications

Astronomy: measuring how fast stars and galaxies move (radial velocity). Detecting exoplanets (the 'wobble' method — a planet's gravity makes its star move slightly, Doppler-shifting the starlight). Measuring the expansion of the universe (cosmological redshift). Medical: Doppler ultrasound measures blood flow (reflected sound from moving blood cells shifts frequency). Used in cardiology, obstetrics (foetal heartbeat), and detecting blocked arteries. Weather: Doppler radar tracks the motion of r