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Homework Help: Simple laser question

  1. Oct 22, 2007 #1
    Hi I'm a student in school and I'm having a mental block on how to do this question. Could some give me a hint as to the principals I'm meant to consider /use.

    Q) A red, 100mW, laser beam , wavelength 650nm, is incident normally on a perfectly black disk . Calculate the force on the disk.

    I've never really had to relate photons to forces before. I tried consider change in momentum of photon is equal to impulse on black disk but that didn't work.

    Thanks for the help
  2. jcsd
  3. Oct 22, 2007 #2


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    You are correct it's just the momentum of the photons ( = h / lambda )
    You can treat the photons just like you were throwing tennis balls at the disc - but remember that the photons are absorbed by the black disc so it is an in-elastic collision.
  4. Oct 22, 2007 #3
    i tried using change of momentum = impulse but I only need to use lambda or Power not both so i think I'm doing something wrong can someone show me a solution.

  5. Oct 22, 2007 #4


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    You need both, the power tells you how many photons/sec hit the disc and the wavelength tells you how much effect they have.
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