Doppler Effect in Astronomy

speed camera




Because EM-waves have wavelike properties, they behave in principle in the same way as sound waves as regards the Doppler effect.

We all know the risk of getting caught speeding by a camera on the side of the road. Our speed is measured with radio waves or infrared light using the Doppler effect.







In astronomy we can observe line spectra from all celestial objects that are radiating, even in wavelengths outside the visible spectrum. We discussed this extensively in our EBook Stellar Radiation. 

These emission or absorption lines are ideal objects to use for the Doppler effect. Astronomers know the line spectrum for a given chemical element from the laboratory. When the celestial object is moving away from us, the frequency becomes lower (compare with lower frequency when the race car is receding), thus longer wavelength. The lines are horizontally shifted towards the longer wavelength. In the visible spectrum that is towards the red side of the spectrum. Therefore this shift is called a redshift. Alternatively, when the celestial object is moving towards us, the lines will be blue shifted.


Because these line spectra can be observed with great detail using modern spectrographs, the Doppler shift can be measured very accurately, revealing the relative (line-of-sight) speed (or radial velocity) between the celestial object and us here on Earth as observer.



Ignoring relativistic effects (which must be taken into account when the relative speed becomes very large), the radial velocity can be determined from the observed Doppler shift with:
This ratio is called redshift, denoted by z
equation 1