Doppler Shift Calculator (Sound & Light)

Doppler Effect Guide

What is the Doppler Effect?

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, and even gravitational waves, since the same underlying wave physics governs each.

What's the key thing to understand about 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: an ambulance siren at 700 Hz, travelling at 30 m/s toward you, is heard at a noticeably higher frequency than its true pitch, then drops sharply as it passes and recedes.

What do I need to know about 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 universe itself is expanding, stretching the wavelength of light as it travels toward us over billions of years.

What's the key thing to understand about 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 rain and storm systems, allowing meteorologists to detect rotation within a storm that can indicate a developing tornado.