How does light speed back up after going through water?

Answer 1

Try not to imagine light as like a Baseball that slows down as it hits a denser medium (like water), and then (seemingly impossibly) speeding up as it goes back to a less dense medium (like air).

Light should instead be viewed as an electromagnetic wave, a wave that can be altered by the charged particles within a medium. How that EM wave is altered depends on the frequency with which the wave is vibrating and the nature of the charged particles in the medium.

When you do the calculations rigorously, you find that water molecules will cause this wave to propogate less slowly. When it leaves the water into air, almost no charged particles exist to alter the speed of propogation of this wave -- it instead travels at the speed Maxwell said such an EM wave would propogate.

Answer 2

Light slows down when it enters a denser medium like water due to increased interactions with molecules. When light exits the water and enters air again, it returns to its original speed as there are fewer interactions to impede its progress.