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Homework Help: Determine Radiation pressure given the fraction of total absorption

  1. Oct 1, 2013 #1
    1. The problem statement, all variables and given/known data

    Radiation of intensity I is normally incident on an object that absorbs a fraction frac of it and reflects the rest back along the original path. What is the radiation pressure on the object?

    2. Relevant equations

    Radiation pressure Pr (force/unit area)

    Pra = I/c (total absorption)

    Prr = 2I/c (total reflection back along path)

    3. The attempt at a solution

    The object absorbs a fraction frac of the intensity, which I translate mathematically as:

    Pra = (frac)*I / c

    This would mean that the total intensity reflected is:

    Prr = 2*(1 - frac)*I/c

    Unfortunately the answer is: Pra = (2 - frac)*I / c

    I don't know how they came to this conclusion. Thanks in advance, MrMoose
  2. jcsd
  3. Oct 1, 2013 #2
    So how do you go from Pra and Prr to total pressure?
  4. Oct 1, 2013 #3
    Holy smokes. Thanks.

    Pra + Prr = (frac)*I/c + 2*(1-frac)*I/c = I*frac/c + 2*I/c - 2*frac*I/c

    = (2 - frac)*I / c
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