The increase in wavelength of electromagnetic radiation caused either by the Doppler effect, when the source of radiation is moving away from the observer, or by the presence of a gravitational field. It is quantified in terms of the wavelength change Δλ expressed as a fraction of the rest wavelength l measured when the source and observer are not in relative motion: z = Δλ/λ. The theory of the Doppler effect gives the relationship between redshift arising from relative motion and the relative velocity of the source and observer. The redshifts of galaxies and quasars are particularly important in astronomy since, through Hubble’s Law, they are generally regarded as direct indicators of the distances to these objects. In General Relativity, Einstein also predicted that there would be a redshift effect in the presence of a strong gravitational field.