<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> In physics, redshift happens when light or other electromagnetic radiation from an object is increased in wavelength, or shifted to the red end of the spectrum . In general, whether or not the radiation is within the visible spectrum, "redder" means an increase in wavelength--equivalent to a lower frequency and a lower photon energy, in accordance with, respectively, the wave and quantum theories of light . </P> <P> Some redshifts are an example of the Doppler effect, familiar in the change of apparent pitches of sirens and frequency of the sound waves emitted by speeding vehicles . A redshift occurs whenever a light source moves away from an observer . A special instance of this is the cosmological redshift, which is due to the expansion of the universe, and sufficiently distant light sources (generally more than a few million light years away) show redshift corresponding to the rate of increase in their distance from Earth . Finally, gravitational redshift is a relativistic effect observed in electromagnetic radiation moving out of gravitational fields . Conversely, a decrease in wavelength is called blueshift and is generally seen when a light - emitting object moves toward an observer or when electromagnetic radiation moves into a gravitational field . However, redshift is a more common term and sometimes blueshift is referred to as negative redshift . </P> <P> Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns . Redshifts are also seen in the spectroscopic observations of astronomical objects . Its value is represented by the letter z . </P>

The stretching of light waves due to the motion of stars away from the earth is known as what
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