What leads to light being red or blue shifted?

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Light is blue shifted or red shifted when the source of the light and the observer are moving relative to each other. Redshift and Blueshift are the result of a phenomenon known as Doppler effect.

If a source of light and the observer are moving closer to each other the number of wavelengths of light received by the observer per second increases, this is the same as an increase in the frequency of the light wave being emitted as measured by the observer. This is known as Blueshift.

On the other hand, if the source of light and the observer are moving away from each other the number of wavelengths of light received by the observer per second decreases. In turn this leads to a decrease in the frequency of the light wave as measured by the observer. This is known as Redshift.

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