When do we confuse frequencies?


The speed of sound waves as well as the speed of light is not changed by the motion of a source of waves. For example, when a sound source moves towards a listener, each next vibration reaches the listener’s ear at time shorter than the period of sound vibration because waves need to covers a shorter distance due to incoming source motion. The reciprocal to that time is the sound frequency which ears perceive so the sound pitch becomes higher. Unlike light, sound wave propagate in some medium. As such, in the case when the listener moves through the medium toward the source, waves seem to move faster to him so the pitch becomes again higher. A difference is that in the former case frequency increases infinitely if the source moves at the sound speed. The fact that waves bunch together or spread out depending on the direction of motion of their source was first analysed by Christian Doppler in the framework of study of star light colors in 1842. Speed of light remains constant regardless the motion of observer so for electromagnetic waves the two situations above are identical in accordance with the principle of relativity. As the relative speed of the light source and the receiver approaches the speed of light the time dilation takes place so that the wavelength difference rapidly increases. Due to the Universe’s expansion, the distant galaxies run away at speeds increased with their remoteness. So their light wavelength increases and we see them redshifted.


Redshift of a distant galaxy

The light of the most distant galaxy observed was emitted about half million years after the Big Bang. We see its wavelength a dozen times longer then when emitted.


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