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Homework Help: Finding initial wavelength of photo given angle of deflection

  1. May 28, 2014 #1
    1. The problem statement, all variables and given/known data

    Find the initial wavelength of a photon that loses half its energy when it Compton-scatters from an electron and emerges at 90∘ to its initial direction of motion.

    3. The attempt at a solution

    Ei = Ef +KE
    Ei = 0.5Ei + KE

    Using compton's formula for scattering of photon:

    Δλ = λf - λi = 2.42444x10-12 m

    λi = λf - 2.42444x10-12 m

    pretty much stuck. If I know the initia energy, find the initial wavelength would be a breeze but I don't have that information. Or if I know the numerical value of KE, I can by KE = 0.5Ei, find initial wavelength.
  2. jcsd
  3. May 28, 2014 #2


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    What did you do with the 90 degrees information ?

    [edit] Oops, already used. Good. So what does halving the energy do to the wavelength of a photon ?
    Last edited: May 28, 2014
  4. May 28, 2014 #3
    not really sure if I will incur the wrath of the moderators but I already solved it.

    The key was introducing E=mc2 and E = hf
    one learnts that f could be found.
    f is also equals to c/λ and with this λ could be found.
  5. May 28, 2014 #4


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    Well done. No reasons for our good moderating spirits to get excited at all !
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