Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Incident X-ray photon problem

  1. Aug 20, 2007 #1
    1. The problem statement, all variables and given/known data

    An X-ray photon is scattered at an angle of θ = 180.0° from an electron that is initially at rest. After scattering, the electron has a speed of 4.67 x 106m/s. Find the wavelength of the incident X-ray photon.

    2. Relevant equations

    hc/λ = hc/λ′ + KE

    λ′ = λ + h/mc (1-cosθ)

    3. The attempt at a solution

    Ok, I solved the first equation for λ′and set both equal to each other:

    hc/λ = hc/λ′ + KE
    λ′= λ + (λ/hc)KE (hopefully my algebra isn't off...could somebody check?:confused:)

    λ + (λ/hc)KE = λ + h/mc (1-cosθ) (subtract λ from both sides)

    (λ/hc)KE = h/mc (1-cosθ) (multiply both sides 1/KE)

    λ/hc = [h/mc (1-cosθ)]/KE (multiply both sides by hc/1)

    λ = [h∧2/m(1-cosθ)]/KE

    then plug in numbers....
    But, do I have it right so far?
  2. jcsd
  3. Aug 20, 2007 #2


    User Avatar
    Science Advisor
    Homework Helper

    Momentum is conserved, so all the new momentum of the electron came from the photon.
    momentum of the electron = m v
    momentum of the photon = h f = h c / wavelength.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook