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Ray of light travels from an optically denser medium

  1. Aug 20, 2005 #1
    When a ray of light travels from an optically denser medium to a rarer one such that the angle of incidence equals the critical angle, then the refracted ray grazes the interface.

    Now, will the principle of reversibility of light work in this case? If yes, please explain.
  2. jcsd
  3. Aug 20, 2005 #2

    Doc Al

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    Staff: Mentor

    Does Snell's law depend on direction?
  4. Aug 20, 2005 #3
    Yes, it is reversible.

    But the corresponding evansecent wave is somthing that you cannot form
    with an ordinary source of light. If you could, you could launch a planewave
    into the denser medium at the critical angle.
  5. Aug 20, 2005 #4


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    why is that?
  6. Aug 20, 2005 #5
    The evanescent wave is not space-propagating. So there is no way to
    form a planewave of (space-propagating) light which could be directed at the interface from
    the low-index side and result in the right surface fields needed to launch
    the planewave at the critical angle into the dense medium.
    Last edited: Aug 20, 2005
  7. Aug 21, 2005 #6


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    but surely it doenst correspond with snell's law.

    if we have n1=1 and n2=1.33
    then the critical angle from a light medium (n1) to a denser medium (n2) is
    a_cr=arcsin(n2/n1)=arcsin(1.33) and needless to remind you the limits of sin function.
  8. Aug 21, 2005 #7


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    But you are not "running it in reverse". As light goes from a lighter medium to a heavier (higher speed of light in the medium to lower), the "critical angle" is the incoming angle at which the outgoing angle is 90 degrees. That does not mean that, going the other way, a vertical beam will leave at the "critical angle". A vertical beam will leave vertically.
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