Because the Doppler effect was first used with visible light in astronomy, the terms “blueshift” and “redshift” became well established. Today, astronomers use these words to describe changes in the wavelengths of radio waves or X-rays as comfortably as they use them to describe changes in visible light.
The greater the motion toward or away from us, the greater the Doppler shift. If the relative motion is entirely along the line of sight, the formula for the Doppler shift of light is
Δλ /λ = v / c
where λ is the wavelength emitted by the source,
Δλ 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.
The variable v is counted as positive if the velocity is one of recession, and negative if it is one of approach. Solving this equation for the velocity, we find
v = c × Δλ / λ.
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