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Light momentum inside a dielectric medium?

  1. Dec 13, 2007 #1
    Light in a vacuum has momentum [tex]\hbar k[/tex] but when the light enters a dielectric medium, fx. glass, the momentum of the light is increased to [tex]\hbar\sqrt{\epsilon} k[/tex], where [tex]\epsilon[/tex] is the dielectric constant of the dielectric. Where does the extra momentum come from? How can this be in accordance with conservation of momentum?

  2. jcsd
  3. Dec 13, 2007 #2
    I would have argued (rather naively btw) that the momentum is [tex]hv/\lambda[/tex], and since the wavelength and wave velocity both decrease by a factor of [tex]\sqrt{\epsilon}[/tex] that the momentum is not changed. How did you get that [tex]k[/tex] increases by [tex]\sqrt{\epsilon}[/tex]?
  4. Dec 13, 2007 #3


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    There is reflection at the boundary. Conservation of energy requires including the reflected wave. Conservation of momentum need not follow, because the dielectric material can acquire momentum.
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