I think you're talking about "the electron is accelerated at a speed greater than the speed of light in water"? The "in water" is the important bit. Light trave

I think you're talking about "the electron is accelerated at a speed greater than the speed of light in water"? The "in water" is the important bit. Light travels [as measured in a straight line] slower in water due to bouncing off of the water molecules. It's not that light itself is slow it just takes light longer to make it through the water because it takes a longer path.