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Blueshift is a phenomenon observed in astronomy and physics where electromagnetic radiation from an object undergoes an increase in frequency and a corresponding decrease in wavelength. This occurs when the source of the radiation is moving towards the observer. The term "blueshift" is derived from the fact that shorter wavelengths of visible light are towards the blue end of the spectrum.

The effect is a direct consequence of the Doppler effect, which describes the change in frequency of a wave in relation to an observer who is moving relative to the wave source. For light waves, this means that if a celestial object is approaching Earth, the light it emits will be compressed, resulting in a shift towards the blue end of the electromagnetic spectrum. Conversely, if the object is moving away, its light will be stretched to longer wavelengths, causing a redshift.

Blueshift is a crucial tool in astrophysics for determining the motion of celestial objects. By analyzing the spectrum of light from a star, galaxy, or other astronomical source, astronomers can identify specific spectral lines (absorption or emission lines) that correspond to known elements. The precise shift in these lines from their expected laboratory wavelengths reveals whether the object is moving towards or away from us and at what speed.

While redshift is more commonly observed due to the expansion of the universe causing most distant galaxies to recede from us, blueshift is observed for objects within our local group of galaxies and for stars orbiting within our own Milky Way galaxy that are moving towards our solar system. It also has applications in various fields of physics, including the study of particle accelerators and the interaction of light with matter.