# Light travels in a medium

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?

• 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'. Jul 13, 2015 at 13:37
• 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 Jul 13, 2015 at 13:40