According to Snell's law :

$${n_1 \over n_2} = {v_2 \over v_1}$$

$v_2 = v_1 n_1 / n_2$

Assuming that $n_1$ is vacuum , we will find the following equation:

$$v = c / n$$

(We may find the same equation directly according to the definition of refractive index : $v = c /n$)

Theoretically , according to this equation in a certain medium the speed of light can become small and even 0.

1- Under these conditions , will the light will be at rest ?

2- If true , a viewer in such medium can move faster than the speed of light. Theoretically, is such occasion possible ? Under certain conditions , is it possible to move faster than the light?

  • $\begingroup$ Be aware that in the photon picture the individual photons still move at the vacuum speed of light. It is the coherent scattering off the atoms and molecules in the medium that leads to an effectively slower speed of light. Now, this explanation I just gave has a ton of problem of its own which I conveniently evaded by writing 'coherent'. $\endgroup$ Jul 13, 2015 at 13:37
  • $\begingroup$ For a nice layman’s discussion of this point, I refer you to this excellent interview in Brady Haran's Youtube series Sixty Symbols. There's also another video which takes on quite a different point of view $\endgroup$ Jul 13, 2015 at 13:40

1 Answer 1


Light will never be completely at rest, but we have succeeded in slowing it down significantly. (See this for example)

In a medium, particles can move faster than the speed of light. (The speed of light in that medium)
In fact, this is used in some particle accelerators to detect certain particles.
When a charged particle travels faster than the speed of light in a medium, it emits Cherenkov Radiation. (See this for a brief explanation)

  • $\begingroup$ Another description of slowing the speed of light in a Bose-Einstein condensate: news.harvard.edu/gazette/1999/02.18/light.html. $\endgroup$
    – Ernie
    Jul 13, 2015 at 12:29
  • $\begingroup$ Thanks for replying. I am just looking for little more explanation why light will never be completely at rest. Is there particular reason for that ? $\endgroup$
    – Michael
    Jul 13, 2015 at 17:58
  • 1
    $\begingroup$ @Michael Look up this post, if you want deeper analysis of speed of light in medium. $\endgroup$
    – Vaimsus
    Jul 13, 2015 at 18:26

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