When light travels through a medium, even a sparse medium, it slows down from the speed of light in vacuum to the speed of light in that medium. When that light re-enters a true vacuum, it will resume moving at the speed of light in a vacuum.
- Why the light doesn't keep the same speed it had when it was in the prism, when it excite the prism ?
- Is the acceleration instantaneous at the exit of the prism ?
- What makes the light keep going and not slowing down and stopping ?
