The redshift effect must work on all frequencies, so blue would be shifted down to some lower frequency, and ultra violet would be shifted down also, into the visible area. How would you know what part of the (pre-redshifted) spectrum you were looking at?

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    $\begingroup$ Welcome on Physics SE :) as of now, it is a bit hard to understand exactly what you mean - do you think you could edit your post to elaborate a bit more? Thus, it would be easier for people to understand what you mean :) $\endgroup$
    – Sanya
    Apr 20, 2017 at 11:46
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    $\begingroup$ Clear question. You're right, without the comparison to some other phenomenon it's not possible to find the real blue or red shift. The clue is the set of wavelengths for known elements as H, He, C, .... See Fraunhofer spectra. $\endgroup$ Apr 20, 2017 at 12:01
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    $\begingroup$ You're right: There's absolutely no way, at all, to know, unless you are looking at some known marker, like the emission lines. Note that - just as you have worked out in your question - the phrase "red shift" means "shifted towards red". It does not mean "the red is shifted". $\endgroup$
    – Fattie
    Apr 20, 2017 at 16:21
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    $\begingroup$ Possible duplicate of How is Doppler redshift of distant galaxies established? $\endgroup$
    – ProfRob
    Apr 20, 2017 at 17:43

1 Answer 1


Specific wavelengths in a spectrum, whether later shifted or not, are identified by the absorption bands of common elements near the emitters. For example, the spectral absorption lines of hydrogen are well known, and there is pretty much always some hydrogen near where the light was emitted. The absorption lines are shifted, but the various lines relative to each other are still a identifiable signature. It is by looking for these characteristic absorption lines and seeing how much they are shifted that we determine the magnitude of the shift in the first place, most of the time.

  • $\begingroup$ Can you provide more information? $\endgroup$
    – peter
    Apr 20, 2017 at 12:21
  • $\begingroup$ @peter: Wikipedia Hydrogen Line $\endgroup$
    – MSalters
    Apr 20, 2017 at 13:55

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