This is what Mr Tan gave us via email. This practically answers the question about light - from air to water.
When the electromagnetic wave strikes the surface, the oscillating electric field forces the electrons in the molecules of the second medium to oscillate at the same frequency as the wave. The accelerating electrons behave like atomic antennas that radiate “extra” electromagnetic waves, which combine with the original wave. The net electromagnetic wave within the second medium is a superposition of the original wave plus the extra radiated waves, and it is this superposition that constitutes the refracted wave. Since the extra waves are radiated at the same frequency as the original wave, the refracted wave also has the same frequency as the original wave, but has a higher speed.