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B Why doeslight "turn" when it travels through water?

  1. Jul 20, 2017 #1
    This might be a silly question.

    Ever since I can remember, I've been taught that when light crosses the boundary between two mediums with different refractive indices, its trajectory changes due to a reduction or increase in its speed. I've never given this a second thought, but today I realized I have absolutely no clue why this happens.

    My problem is in making the leap from light slows down to therefore it turns. In my mind, slowing down would only mean that it gets somewhere a little later. So if it takes a beam of light 1 nanosecond to get from point A to B in a vacuum, it should take 1.5 nanoseconds if the medium was water, but that's about the only difference I'm conceptualizing as a direct consequence of light "slowing down". Just like if a car were to slow down from 100 MPH to 70 MPH; I would not expect this to cause the car to turn, say, right, by 30 degrees - it would keep moving in the same direction as before, only slower.

    What am I missing?

    Last edited: Jul 20, 2017
  2. jcsd
  3. Jul 20, 2017 #2


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    There are many diagrams available to help you here. This link shows how the wave crests become further apart as the light (or any other wave actually) crosses from a fast medium to a slow medium. There has to be continuity as the wave crosses the boundary - the crests cannot suddenly turn up somewhere else - and the only way is for the crests to be closer to each other and for their angle to change.
    BTW, I googled refraction explanation wave front and got a huge number of hits. Try it.
  4. Jul 20, 2017 #3
    Thank you; the "wave front" part of the search is what I was missing. A regular search for "refraction explanation" turned up mostly simplified explanations that did not show me a direct cause and effect. Much obliged, will read up.
  5. Jul 20, 2017 #4
    Your question is not silly. I wonder if anyone can help without help of maxwells equations.
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