Imagine a mirror is moving away from a light source in a substance through which the speed of light is very slow -- so slow that the speed of the mirror is close to being the same as the speed of the light (in the medium). If light is then emitted and reflected off of the mirror, will the redshift of that light be very dramatic given that the mirror is moving nearly as quickly as the light?

I'm assuming that's an impractical scenario, but I thought it was an interesting question nonetheless.

Thanks in advance!

  • $\begingroup$ I fear there may be a confusion of speed and frequency here. If the reflected light exits the medium (back into, say vacuum), the redshift will just be whatever you'd get from the mirror's speed. $\endgroup$ Commented Jan 15, 2015 at 15:19
  • $\begingroup$ Interesting question. If the refraction index is so big, the refraction angle will be very small s.t. the light will pass through the medium along the normal on the surface, and the wave-length of the light will be very big. As to the reflection, when the phase-plane advances through the medium together with the mirror, I don't see how the mirror can reflect it. But I am not sure. $\endgroup$
    – Sofia
    Commented Jan 15, 2015 at 15:42
  • $\begingroup$ Thanks @CarlWitthoft , so assuming the speed of the mirror is minuscule compared to the speed of light in a vacuum, you're saying that if blue light in a vacuum hits the medium and then the moving mirror (which is presumably moving nearly as fast as the light in the medium at that point), the reflected light will still be blue in the vacuum? $\endgroup$
    – Eric Czech
    Commented Jan 15, 2015 at 18:37
  • $\begingroup$ Yep, the speed of light, and its wavelength, "revert" to what they oughtta be any time the photon is in vacuum. $\endgroup$ Commented Jan 15, 2015 at 20:38
  • $\begingroup$ Aha, exactly the info what I was after then. Thanks! $\endgroup$
    – Eric Czech
    Commented Jan 16, 2015 at 16:54


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