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Violation of law of conservation?

  1. Aug 11, 2005 #1
    violation of law of conservation???

    When the light goes through a glass window, it undergoes refraction inside the glass material and its velocity is decreased (changed)for sure, then how come the light,after coming out of the glass, can travel with the standard velocity?? what is the source that enables the light to gain its speed back? Don't you think here the law of conservation of energy is violated??
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  3. Aug 11, 2005 #2

    Claude Bile

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    The laws of reflection and refraction are derived from the fact that energy and momentum are both conserved when a photon moves through an interface.

    Conservation of energy demands that the frequency must be invariant when travelling through an interface, that is why frequency is always constant in these situations. Only velocity and wavelength change as one moves from one region of refractive index to another.

  4. Aug 11, 2005 #3


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    Staff: Mentor

    On the microscopic scale, the speed of light does not change in refraction - the apparent change in the speed of light comes from absorption and re-emission - otherwise, the photons travel at C.
  5. Aug 12, 2005 #4


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    You find this inconsistency because you are thinking of light as a particle. Remember, light can behave like both :smile:

    Although the analogies are not exactly the same, I think it is nice to compare this with typical ocean waves. Water waves travel faster in deeper areas than they do in shallow. If they pass through unlevel ocean floor, their speed will change accordingly. No violation of energy here.
  6. Aug 12, 2005 #5
    the qm description (absoption/re-emission), which is the correct explanation, doesn't require you to think in terms of wave-particle "duality".
  7. Aug 12, 2005 #6


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    Just for arguments sake, which I know I shall regret later, using the wave description of light one will also avoid having to use the absorption/re-emission explanation.
  8. Aug 12, 2005 #7

    Devide whole situation into three consecutive parts. First before the light enters the glass. Second, while the light is traveling through glass. Third after the light come out of the glass. The first and the third, the light travles with same speed therefore they have equal energy. During the second part, although light seems to slow down, it will certainly increase the energy of the glass via heat and it might as well be that when sum all the energy it will be equal to that of first and the third parts. No?
  9. Aug 13, 2005 #8
    The classical energy of the light is not related to the speed, but the square
    of the field amplitude. In materials there is a more complicated expression
    involving the electric and magnetic polarizabilities of the materials. In other
    words, the fields make the materials vibrate and this energy must be included
    as well.

    You will find if you work this through carefully that energy is conserved
    (if you include where it is dissipated or reflected) at every step of the way.
    Last edited: Aug 13, 2005
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