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Homework Help: Energy of an X-ray photon

  1. Apr 22, 2010 #1
    1. The problem statement, all variables and given/known data
    The X-ray spectrum of molybdenum has a particular characteristic spectral line of
    wavelength 6.6 *10^–11 m. The ionisation energy of molybdenum is 20 keV.

    Deduce that the energy of an X-ray photon of wavelength 6.6*10^–11 m is 19 keV.

    2. Relevant equations
    h=planks constant
    c=speed of light

    3. The attempt at a solution
    Normally this is quite easy. You just plug in wavelength into the equation and it should give you the answer. However, the part that I do not understand is that in the markscheme the proper formula to use is E(ev)=(h*c)/(l*e). Were e is elementary charge. Now this does not make sense to me as (h*c)/(l*e)=V (Voltage). So could someone explain why I need to use elementary charge.
  2. jcsd
  3. Apr 22, 2010 #2


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    Homework Helper

    Physicists often use eV as unit for energy instead of joule. 1 eV is the energy an electron gains when it is accelerated across 1 V potential difference. This energy is equal to 1( V ) * e (charge of electron) = 1.6 10-19 J.

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