In 1842 Christian Doppler hired a group of musicians to play on an open train carriage as it was moving on the track and he heard a difference in tones as they approached him and then receded from him. He applied what he learned to all waves and pointed out that if a light source is approaching or receding from the observer the light waves will be more or less crowded together. The Doppler effect is only produced by a motion toward or away from the observer - called radial velocity. Sideways motion does not produce this effect. An easily observed example is when an emergency vehicles siren approaches and then passes you.
The Doppler effect is observed with all electromagnetic waves, including visible light. The formula for the Doppler shift of light is:
∆𝞴⁄𝞴 = ⱽ⁄c
where 𝞴 is the wavelength emitted by the source, ∆𝞴 is the difference between 𝞴 and the wavelength measured by the observer, c is the speed of light and v is the relative speed of the observer and the source in the line of sight. V is positive if the velocity is one of recession and negative if it is one of approach.
Therefore for the velocity v = c ∆𝞴⁄𝞴
If a star is approaching or receding from us the wavelengths of its light are shortened or lengthened, however unless its speed is tens of thousands of kilometres per second its colour does not perceptively change. However the wavelengths of the absorption lines can be accurately measured making the detection of their Doppler shift relatively simple.
When identifying the elements in stars astronomers do not look at one coloured spectral line, rather the patterns of spectral lines associated with each element. The Doppler effect does not change this pattern, rather shift all the lines redder or bluer.
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