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Homework Help: X-Ray photon/wavelength question

  1. Apr 1, 2006 #1
    This is one of the questions on my regular worksheets we have before our tests:

    A fast-moving electron travelling through a vacuum tube slams into a piece of steel, coming abruptly to rest and emitting an X-ray photon with an energy of 7.90×10-16 J. What is the wavelength of the photon?

    I know I have to find the frequency and speed of the photon in order to find the wavelength. The problem is I do not know how to find either of the two (I had a long day at work so I'm pretty sure I'm just having a brain fart right now). If someone could help point me in the right direction it would be more than appreciated!
     
  2. jcsd
  3. Apr 1, 2006 #2

    Pengwuino

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    Photons always travel at 3x10^8 m/s, the speed of light.
     
  4. Apr 1, 2006 #3

    dav2008

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    For a photon:
    [tex]E=hf[/tex]
    [tex]E=[/tex]Energy of the photon
    [tex]h=[/tex]Planck's Constant
    [tex]f=[/tex]Frequency

    and of course for any wave [tex]v=f \lambda[/tex], and like Pengwuin said in the case of photons your velocity is about 3x108 m/s
     
  5. Apr 1, 2006 #4
    Edit: Thank you dav, I thought I was on the right track but you confirmed it. Thanks a bunch guys!
     
  6. Apr 1, 2006 #5
    Okay, I think I had better post again, because something is surely off in what I am doing with this problem. My answer that I got for Frequency was something in the neighborhood of 1.16x10^-50 s, and whenever I plug that into the wavelength equation and solve for wavelength I get the astronomical number of 2.58x10^58 cm/s.

    Am I doing this problem right and should be getting a number that high? Or am I off in some of my calculations?
     
  7. Apr 1, 2006 #6

    dav2008

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    Look at the E=hf equation more carefully... I think you multiplied instead of divided.

    Also keep in mind that wavelength has dimensions of length.
     
  8. Apr 1, 2006 #7
    to find f I've been putting in f = E/h, which I've been imputting as the energy from the original problem, 7.9 x 10^-16, divided by h, so it looks like 7.9 x 10^-16/6.626 x 10^-34. Is this wrong right here or is my calculator possibly giving me funky numbers for no reason? lol.
     
  9. Apr 1, 2006 #8

    dav2008

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    [tex]\frac {7.9 \cdot 10^{-16}(J)}{6.626 \cdot 10^{-34}(J \cdot s)}[/tex] gives me a frequency of [tex]1.19 \cdot 10^{18} (\frac {1}{s})[/tex].
     
    Last edited: Apr 1, 2006
  10. Apr 1, 2006 #9
    yeah, I realized i was putting something wrong into the equation, so for wavelength I got .252 cm...which corresponds to 2,520,000 nanometers...is this close to being right or am I still screwing something up?
     
  11. Apr 1, 2006 #10

    dav2008

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    You divided 3x108m/s by 1.19x1018 1/s and got .00252 meters?

    Check your calculations.
     
  12. Apr 1, 2006 #11
    Okay, I redid everything and I got 2.521x10^-9 m for the wavelength...I might as well take baby steps since either I or my calculator is screwing up what I type, so is this calculation correct? And if so, to convert it to nanometers I'm supposed to divide by 10^-9 if I'm not mistaken.
     
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