Named after the Austrian who in 1842 tried to explain the colours of double stars (1842)
Christian Doppler's paper 'Über das farbige Licht der Doppelsterne'
Science · Physics
The Doppler effect
1842
Physics — waves and acoustics
The change in the observed frequency of a wave when the source and the observer are moving relative to one another. Proposed by the Austrian physicist Christian Doppler in an 1842 paper on the coloured light of double stars, it is the reason a siren drops in pitch as an ambulance passes, and the reason light from receding galaxies is shifted towards the red end of the spectrum. It underpins weather radar, police speed guns, medical ultrasound, the discovery of exoplanets by the wobble of their stars, and the evidence for an expanding universe.
How to spot them
- ▸Why the pitch of a passing ambulance siren drops as it goes by
- ▸The reason light from receding galaxies is shifted towards the red
- ▸Tested in 1845 by a Dutch meteorologist using trumpeters on an open railway carriage
- ▸The principle behind speed guns, weather radar and foetal heart monitors
Key facts
Trumpeters on an open railway carriage between Utrecht and Maarssen, 1845 (1845)
C. H. D. Buys Ballot set out to disprove the idea and confirmed it instead
The falling pitch of a passing ambulance siren
The everyday demonstration: waves bunch up ahead of the source and stretch out behind it
The redshift of receding galaxies, and Hubble's expanding universe
The cosmological version, in which stretched light reveals recession velocity
The wobble method for detecting exoplanets, and the police speed gun
Radial-velocity searches and radar guns both measure a frequency shift
The French physicist Fizeau independently described the same shift for light in 1848 (1848)
Hence 'Doppler–Fizeau effect' in French usage
Key ideas
- A moving source crowds its wavefronts together in the direction of travel and spreads them out behind, so an approaching source sounds higher and a receding one lower.
- The observed frequency changes, not the emitted one — the siren's note is constant to the driver.
Quick-fire facts
- •Named for Christian Doppler (1803–1853), who announced it in Prague in 1842 in a paper on the colours of double stars
- •His stellar-colour explanation was wrong — stars are the wrong colour for that reason — but the underlying principle was right
- •Tested in June 1845 by Christophorus Buys Ballot on the Utrecht–Maarssen railway, with trumpeters on a flatcar; the note rose roughly a semitone on approach
- •Buys Ballot ran the experiment expecting to refute Doppler, and also gives his name to Buys Ballot's law about wind and pressure
- •Hippolyte Fizeau independently treated the effect for light in 1848, so French sources call it the Doppler–Fizeau effect
- •Redshift is the astronomical version; Edwin Hubble used galactic redshifts in 1929 to show the universe is expanding
- •Cosmological redshift is strictly the stretching of space, not motion through it — a common misconception
- •The effect is relativistic for light: there is also a transverse shift with no approach or recession at all, from time dilation
- •Everyday applications include Doppler weather radar, ultrasound blood-flow imaging, laser Doppler velocimetry and speed cameras
- •The Doppler crater on the Moon and the asteroid 3905 Doppler are named after him
Drill it — 8 cards
1 / 8new card
Sources
- Physics Today, American Institute of Physics — The fall and rise of the Doppler effect
- KNMI (Royal Netherlands Meteorological Institute) — Doppler-experiment Buys Ballot, 160 jaar geleden On the 1845 railway test