A223: The Doppler shift (or Doppler Effect) is an increase or decrease in wavelength as the object emitting the wave moves relative to the observer. For example, a train whistle seems to be higher in pitch when the train is approaching you (the waves are compressed, shortening the wavelength), and lower in pitch when it is traveling away from you (the waves are elongated, lengthening the wavelength). The same thing happens with light waves when the light source is coming or going relative to us. For example, when a star is travelling away from Earth, its light appears redder (the light waves are elongated, lengthening the wavelength); this is called the red shift. The expansion of the universe was discovered when E. Hubble observed that the light from almost all other galaxies was red-shifted. The Doppler effect was named for Johann Christian Doppler (November 29, 1803-March 17, 1853), who first realized that it existed (1842).
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