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.

  1. Why the light doesn't keep the same speed it had when it was in the prism, when it excite the prism ?
  2. Is the acceleration instantaneous at the exit of the prism ?
  3. What makes the light keep going and not slowing down and stopping ?

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