In order to calculate the redshift one has to know the wavelength of the emitted light in the rest frame of the source, in other words, the wavelength that would be measured by an observer located adjacent to and comoving with the source. Determining the redshift of an object in this way requires a frequency- or wavelength-range. If the same spectral line is identified in both spectra-but at different wavelengths-then the redshift can be calculated using the table below. If the same pattern of intervals is seen in an observed spectrum from a distant source but occurring at shifted wavelengths, it can be identified as hydrogen too. If restricted to absorption lines it would look similar to the illustration (top right). The spectrum of originally featureless light shone through hydrogen will show a signature spectrum specific to hydrogen that has features at regular intervals. A very common atomic element in space is hydrogen. If found, these features can be compared with known features in the spectrum of various chemical compounds found in experiments where that compound is located on Earth. To determine the redshift, one searches for features in the spectrum such as absorption lines, emission lines, or other variations in light intensity. The spectrum of light that comes from a single source (see idealized spectrum illustration top-right) can be measured. There exist other physical processes that can lead to a shift in the frequency of electromagnetic radiation, including scattering and optical effects however, the resulting changes are distinguishable from true redshift and are not generally referred to as such (see section on physical optics and radiative transfer). Special relativistic, gravitational, and cosmological redshifts can be understood under the umbrella of frame transformation laws. However, in many contexts, such as black holes and Big Bang cosmology, redshifts must be calculated using general relativity. Its value is represented by the letter z.Ī special relativistic redshift formula (and its classical approximation) can be used to calculate the redshift of a nearby object when spacetime is flat. Redshifts are also seen in the spectroscopic observations of astronomical objects. Knowledge of redshifts and blueshifts has been applied to develop several terrestrial technologies such as Doppler radar and radar guns. However, redshift is a more common term and sometimes blueshift is referred to as negative redshift. 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. Finally, gravitational redshift is a relativistic effect observed in electromagnetic radiation moving out of gravitational fields. 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. A redshift occurs whenever a light source moves away from an observer. 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. 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. 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.
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