# 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 '14 at 9:27

Light is fastest in Vacuum which is least dense medium in the universe. – SS-3 Mar 29 '14 at 3:36
Possible duplicates: physics.stackexchange.com/q/466/2451 , physics.stackexchange.com/q/11820/2451 and links therein. – Qmechanic Mar 29 '14 at 7:57
Kerosene is lighter than water but light travels faster in it. – evil999man Mar 29 '14 at 8:18
Also, see here youtube.com/watch?v=FAivtXJOsiI – Iota Mar 29 '14 at 9:09

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).