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Radiation pressure, momentum question.

  1. Jul 21, 2010 #1


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    If an electromagnetic radiation passes through a transparent medium (dielectric or conductor), and if the radiation creates a pressure upon that medium, that means that a part of the photons momentum will be transferred to the medium and the radiation will exit with a higher wavelength ?
    How exactly momentum is transferred to transparent materials? Thanks!
    Last edited: Jul 21, 2010
  2. jcsd
  3. Jul 22, 2010 #2
    Hi there,

    No material is completely transparent (except for vacuum of course).

  4. Jul 22, 2010 #3
    Objects feel radiation pressure when they absorb light. A perfectly transparent material should feel no radiation pressure, since it doesn't absorb light.
  5. Jul 23, 2010 #4
    And if there happens to be any absorption then in most cases the light exiting the material has fewer number of photons, but unchanged wavelength, i.e. any momentum transfered to the medium is due to removal of whole photons from the beam, and not due to each photon changing wavelength slightly.
  6. Jul 23, 2010 #5


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    You should not forget that part of the light is reflected when entering the medium neccessarily when the indices of refraction do not coincide.
  7. Jul 23, 2010 #6
    But if the material is completely transparent, than no light is reflected.
  8. Jul 23, 2010 #7


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    No, transparency means that the imaginary part of the index of refraction is zero.
    For perpendicular incidence from vacuum onto a medium of refractive index n, the reflectivity is then given as R=(n-1)^2/(n+1)^2.
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