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The Doppler Effect states that the frequency of sound and light emanating from a point will change depending on the motion of the source relative to an observer.  For example, a bystander hears the pitch of an approaching train change as it passes and moves away from him.
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The Doppler Effect states that the frequency of sound and light emanating from a point will change depending on the motion of the source relative to an observer.  For example, a bystander hears the pitch of an approaching train whistle change as it passes and moves away from him.  As the train approaches, the pitch of the whistle increases, and then decreases after the train has passed by.
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This was discovered in 1842 by the Austrian Christian Johann Doppler.
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In the case of light, when the observer and light source approach each other, the the wavelength of the light decreases, shifting the observed light to the blue end of the [[spectrum]], known as a blue shift.  Conversely, when the observer and light source move away from each other, the light shifts to the red end of the spectrum, known as a red shift.     
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The effect was named after Austrian [[Christian Johann Doppler]], who provided the first technical explanation of the phenomenon in 1842.
    
[[Category:Physics]]
 
[[Category:Physics]]
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