We know, As per Doppler effect, if an object travels with speed close to that of light, if appears red if it goes away from observer and it appears blue if it comes towards the observer.

My question is what will happen if

  1. The object itself is of red color and going away from observer nearly with speed of light

  2. The object is of blue color and coming towards the observer nearly with speed of light.

This is my first day on this platform. I am sorry if i have typed anything wrong.

  • $\begingroup$ Things that are too red or too blue cannot be seen by human eyes and are black like ultraviolet and infrared. It's just a matter of shorter or longer wavelengths. $\endgroup$
    – DKNguyen
    Jul 16, 2021 at 3:09
  • $\begingroup$ Blue will be Rontgen or even hard gamma. Red will become radio. The other way round then red can be blue or blue can be red. $\endgroup$
    – user307025
    Jul 16, 2021 at 3:16
  • $\begingroup$ To clarify, an object does not "become red" if it is approaching (unless it is white to start with); rather its frequency is reduced. Nor does it "become blue" if receding; its frequency is increased. You should now be able to answer your own question. $\endgroup$
    – m4r35n357
    Jul 16, 2021 at 9:49

2 Answers 2


"Redshift" is really an idiom for "increase in wavelength" and blueshift means "decrease in wavelength. They get their names because red is the largest wavelength of the visible spectrum and blue (near) the shortest.


The Doppler effect will shorten any wavelength from a source moving towards the observer (blue shift) and lengthen any wavelength moving away from the observer (red shift). This is true whether the source is emitting wavelengths anywhere within, above, or below the visible spectrum.


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