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Light speed in a material

  1. Nov 1, 2013 #1
    Everyone knows that if a light come in a material will change speed(will become less),and from wikipedia I read that light wants to go from his faster way. So when material 'eat' from light some speed,then light as more 'clever',goes from a sooner way.(that what we call "refraction.").

    So my question is: Why light is 'clever?
    Or maybe better: How light change way,when has less speed??

  2. jcsd
  3. Nov 1, 2013 #2


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    For transparent substances (glass, air) we know that an image is propagated by the light passing through that material. For this to happen several things have to be true:

    1. The waves must retain their relative phases (coherence)
    2. The waves must travel in the forward direction (forward scattering)

    Forward coherent scattering only occurs for the light which obeys Snell's law while traveling through the material; Snell's law depends upon the angle of incidence, and the index of refraction for the materials (e.g., air-glass-air).

    For any wave propagation passing from one material to another there is a "Snell's Law"; it is a property of waves, and the change in speed of wave propagation is a measure of "impedance" of the material. This impedance is due to "forward coherent scattering" which delays the phase-front due to interference effects from the different scattering sources. All directions which are not the "forward" direction described by Snell's law result in destructive interference; in the forward direction you get constructive interference.

    So inside the material the light travels at c, but due to the "forward coherent scattering" a geometric analysis shows that the effective speed of the phase front is reduced ... but once the light crosses out of that material the effects are determined by the new material (air after coming from the glass), and the light has a new characteristic speed. But all along it is travelling at c.

    Richard Feynman explains this in more detail in "QED: The Strange Theory of Light and Matter".

    You can find some more detailed information here:

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