Why does the light travel slower in denser medium? 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?
 A: This is quite a subtle issue. The charges in the medium produce secondary spherical expanding EM waves when hit by the primary wave (external forces). There is immense number of these secondary waves. At any point of space, each secondary wave has slightly different wave vector. In a medium dense enough, these secondary waves add to the primary wave in such a way that the resulting wave has behaviour that is well described by a single macroscopic wave of the same frequency and (usually) same direction but (for most frequencies) with a reduced wavelength.
A common picture backed by successes of dispersion theory is that the relation $\mathbf j(t) = c\mathbf E(t-\Delta t)$ is valid, where $c, \Delta t$ are some medium property constants that depend on frequency of the wave, $\mathbf j$ is current density and $\mathbf E$ is total macroscopic electric field. With this assumption, Maxwell's equations imply that the resulting wave in the medium will have modified (in usual cases shorter) wavelength hence lower velocity (for a certain limited interval of frequencies it can have a longer wavelength and higher velocity).
A: 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.
