Doppler effect


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A measurable shift in the wavelength of a traveling wave caused by the relative motion of the source and observer. If a source and observer are approaching each other, the wavelength of incoming light is shifted to shorter, or bluer, wavelengths; if a source and observer are receding from each other, the wavelength of incoming light is shifted to longer, or redder, wavelengths. Stars with relative motion toward an astronomer appear bluer than they would otherwise, i.e. the entire spectrum of star light shifts toward the blue end of the spectrum or is blue-shifted. Stars speeding away from the astronomer appear redder than they would otherwise, i.e. the entire spectrum is red-shifted. The expansion of the universe can be detected because the farther away a celestial object is from the Earth, the more it tends to be red-shifted.