The velocity of light changes between mediums, but does light specifically decelerate when it enters a denser medium, or is the change is speed instantaneous?
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1Light still travels at c, it just interacts with the components of the medium in such a way that the phase velocity of the photons slows down. You can also have the phase velocity exceed light speed in materials with a negative refractive index, but this doesn't mean the signal velocity is above c. What I'm saying is the information may travel at less than light speed, but each individual photon still travels at c. – Jim May 08 '15 at 16:51