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Fermat principle and light

  1. Nov 24, 2008 #1
    In Feynman lectures, Feynman says if light goes from air to denser medium,light goes more in air and less in denser medium so that it takes shortest time to reach its destination.

    My question are

    How light knows that there is a denser medium ahead in its path?

    Does light get information about the denser medium?

    what is information?
  2. jcsd
  3. Nov 24, 2008 #2


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    Gold Member

    Feyman's own explanation is that the light goes everywhere, but only the shortest-time path is not interfered away - in other words only the shortest-time path survives destructive inteference. This seems to require some non-local phenomenon to take place.
  4. Nov 25, 2008 #3
    Yes, i understand shortest time path is the only one surviving..but how does light know its destination and making its path survive from interference? Does light get information about medium and destination?
  5. Nov 25, 2008 #4


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    Science Advisor

    It doesn't. That was what Mentz114 just said.
  6. Nov 25, 2008 #5


    Staff: Mentor

    Refraction is a general property of solutions to the wave equation in different media. The wave equation is purely local, therefore no non-local information is required.

    You can certainly describe the solution in terms of a photon asking yahoo maps to plot it the optimal route, but you don't need to do that. The optimality automatically falls out of solutions to the purely local wave equation.
  7. Nov 25, 2008 #6
    I'd say while it's experimentally known that light takes the least time path, in general the less dense medium, the precise interaction is one of many uncertainities/unknowns in physics. We have insights, as posted, maybe we can call them working rules, but they are not fully "satisfying".

    As for "information", check out Wikipedia for an introduction. Entropy and thermodynamics can be considered as sub categories of information. In Claude Shannons information theory, a "bit" becomes the fundamental unit and in quantum theory, a qubit.

    An interesting book is by Charles Sief: DECODING THE UNIVERSE,2006 (How the new science of information theory is explaining everything in the cosmos...)...even What is life? Does entanglement explain telepathy? and so on.
  8. Nov 26, 2008 #7
    I think Fermat's principle comes from Newton's first law that " all objects in uniform motion tends to be in uniform motion"..so that objects reach their destination in straight line path in least time..For objects to take other paths, a force has to be applied and this will change its uniform motion and they cannot reach their destination in least time..same applies to Light as well..Light also travels in uniform motion and tends to travel in uniform motion..Is this correct?
  9. Nov 26, 2008 #8


    Staff: Mentor

    That is an aestetic judgement. I find Maxwell's equations, the resulting wave equation, and their solutions fully satisfying.
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