That’s Maths: The Doppler effect has numerous practical applications, including in astronomy, aviation, policing and medicine
Doppler radar can determine the radial speed of moving targets. Speed cameras and laser guns can, from a fixed position, catch drivers violating the speed limit. Photograph: Cyril ByrneWe have all noticed how the horn of a speeding car changes as it approaches: each wave-peak is emitted from a closer point, so the wave is “squeezed” and the pitch increases. As the car recedes, the reverse effect stretches the wave, making it sound lower.
A few years later, the Dutch meteorologist impressively named Christophorus Henricus Diedericus Buys Ballot confirmed the same effect for sound waves. Buys Ballot is better known for his law relating atmospheric pressure and wind direction, one formulation of which is “stand with your back to the wind; if low pressure is to your left, you are in the northern hemisphere”.
With Puma equipment, moving Garda cars can detect speeding drivers. Sonar devices are radars using sound waves instead of radio waves. A sonar current profiler measures the speed of currents in rivers and the ocean by detecting the Doppler effect on sound waves scattered from particles in the water. Sodar, which stands for sound detection and ranging, and which measures wind speeds, works on similar physical principles.
The Doppler frequency shift is invaluable in astronomy. The light emitted from stars is in discrete spectral lines and the effect causes small but detectable shifts in these lines . This can tell us how fast stars and galaxies are approaching or receding from us, distinguish close binary systems from single stars and detect exoplanets, which are planets rotating around other stars.
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