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Doppler in light
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Redshift and blueshift reveal motion of stars.
Δλ/λ = v/cFor light, redshift z = Δλ/λ ≈ v/c gives the radial speed of stars and galaxies. Expansion of the universe shows up as cosmological redshift; police radar uses the Doppler shift too.
A galaxy with z = 0.1 recedes at about 30,000 km/s.
At relativistic speeds use the exact formula.
In more detail
The Doppler effect for light shows a redshift for objects moving away and a blueshift for objects moving closer. It is used to measure the speed of stars and galaxies.
Real-world uses
- Astronomers measure galaxy speeds from redshifts, revealing the expansion of the universe.
- Speed guns and lidars use the Doppler effect in light or radio.
- Astronomers measure the redshift of distant galaxies to find how fast they are moving away from us.
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Frequently asked questions
What is redshift?
An increase in wavelength, which moves light towards the red end of the spectrum.
Does redshift always mean motion?
In cosmology, it also reflects the expansion of space, not only motion through space.
How is Doppler shift in light different from sound?
Light's Doppler shift depends only on the relative speed, so it does not need a medium, while sound depends on the speed of the air.
Related topics in Waves & Sound
Doppler effect · Doppler with moving source · Doppler with moving observer · Shock waves and sonic boom · Mach number · Energy transport in waves