A25 The Redshift and the Doppler Effect

The redshift is the name given to the stretching of light waves, or any other electromagnetic waves, produced by movement of the source of the radiation away from the observer. To illustrate this, recall the sound of a jet plane. When it is flying toward you the sound is high-pitched. As the plane passes and flies off into the distance the sound moves to a lower and lower pitch. This is known as the Doppler effect, after the Austrian physicist (Christian Doppler) who first studied the phenomenon of the change in frequency of waves from a moving source. When the plane flies away the sound waves it emits are stretched—that is, their wavelength appears greater That produces a lower tone. Similarly, when a galaxy or star is receding from the earth its light waves are slightly stretched, which means they shift toward the longer, or red end, of the spectrum—hence a redshift. The opposite effect, produced by the object coming toward you. would produce a shortening of the waves, or a blueshift,

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