A Doppler shift (or Doppler effect), in astronomy, is a change in electromagnetic radiation (EMR) frequency and wavelength due to a radial velocity between the observer and the observed, and is a key means of determining radial velocities of astronomical objects. It is a general property of waves, popularly known as the mechanism by which the sound of a passing vehicle has a higher pitch as the vehicle approaches and lower once it has passed. The relationship between wavelength and the relative velocity between the wave source and recipient are:
Δv Δf = —— s
Redshift is the EMR Doppler shift from a source that is receding (the wavelength is lengthened, which shifts visible light toward the red end of its spectrum), and in astronomy, using Hubble's law, is used to measure the distance to very distant galaxies. Gravity also produces a Doppler shift, as outlined by general relativity (GR): when this gravity effect is significant, it can offer information regarding the object under study, but also is a factor that can require accommodation when the Doppler shift is used for the above purposes.