# Why does the light travel slower in denser medium? [duplicate]

Wikipedia says that "in general, the refractive index of a glass increases with its density." And the refraction index of water vapor is less than ice, and even less than liquid water. Is there any simple explanation to that?

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## marked as duplicate by John Rennie, Qmechanic♦Mar 29 at 9:27

Light is fastest in Vacuum which is least dense medium in the universe. –  Sachin Shekhar Mar 29 at 3:36
Possible duplicates: physics.stackexchange.com/q/466/2451 , physics.stackexchange.com/q/11820/2451 and links therein. –  Qmechanic Mar 29 at 7:57
Kerosene is lighter than water but light travels faster in it. –  Awesome Mar 29 at 8:18
Also, see here youtube.com/watch?v=FAivtXJOsiI –  Iota Mar 29 at 9:09

The simplest picture is that light always travels at the speed of light. But in a material it travels at the speed of light until it hits an atom. It is then absorbed and re-emitted in the same direction, which takes a small amount of time.
The more this happens, the slower the effective average speed.
The denser the material, the more atoms there are in the way.

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Great explanation. –  boyfarrell Mar 29 at 6:04
The explanation is only apparent. In classical EM theory there is no time lag between interaction of external EM wave with charges and emission of secondary EM wave, both occur simultaneously. –  Ján Lalinský Mar 29 at 8:02

This is quite a subtle issue. The more charges medium has in unit volume, the more it produces secondary EM waves. Common belief backed by the success of dispersion theory is that the relation $\mathbf j = c\mathbf E$ is valid, where $c$ is a constant dependent on the frequency of the wave, $\mathbf j$ is current density and $\mathbf E$ is total macroscopic electric field (all complex phasors). Maxwell's equations then imply that the resulting wave in the medium will have shorter wavelength hence lower velocity (for certain interval of frequencies it can have longer wavelength and higher velocity).

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