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X-ray tube Questions - Accelerating electrons, Why X-rays

  1. Aug 22, 2011 #1
    Hey guys,

    I have a couple of questions regarding X-rays and how they are produced.

    1) Is an X-ray an electromagnetic wave or a photon?

    2) Why is it that when the electrons collide with the anode in an x-ray tube, X-rays are emitted and not any other electromagnetic wave/photon?

    3) When the electrons are being accelerated from cathode to the anode by the potential difference applied across the X-ray tube, how come they don't emit an electromagnetic wave? According to maxwells theory of electromagnetism, electromagnetic waves are emitted when an electron is accelerating. And isnt this how electrons emit electromagnetic waves in antennas? So how come these ones dont?

  2. jcsd
  3. Aug 23, 2011 #2

    (Remember that visible light is a photon _and_ a transverse eletromagnetic wave, depending on how you measure it. The same is true for other rays. An x-ray is made of photons, and, therefore, is an e-m wave.)

    It is difficult to answer these types of questions without access to the Internets.

    [See: http://en.wikipedia.org/wiki/X-ray] [Broken]


    Apparently, Roentgen had to put cardboard around his cathode ray tube and paint the screen opaque in order to conceal the other electromagnetic radiation that was visible. In fact, the X-rays are visible to the dark-adjusted naked eye if the energy level is sufficient.

    I sorry that I don't know why you claim they don't get emitted. This is exactly how the X (or "unknown") particle/wave is emitted for diagnostic imaging. But visible light and many other wavelengths of radiation do not go through solid objects, such as skulls or mamary glands.

    Last edited by a moderator: May 5, 2017
  4. Aug 23, 2011 #3
    Hey, thanks for the answer.

    With question 3), the X-rays get emitted when they collide with anode, wright? Not when they are being accelerated as far as i have been taught. If it is emitted while they are being accelerated, then how come they need to collide the electrons with the anode and all that?

    Thanks :)
  5. Aug 23, 2011 #4


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    Both models can be applied here.

    The frequency of emitted radiation depends on the acceleration. If they would hit the anode at lower speed, they would emit lower frequencies.

    They do, but the magnitude of acceleration is much lower here, compared to when they hit the anode. So it is a much lower frequency, not X-rays.

    If found a nice animation on this here:


    This is what happens with the E-field when an electron suddenly starts moving, with short period of large acceleration. As you see high frequency (short wave length) radiation goes mostly perpendicular to the direction of acceleration.

    In the X-ray tube the acceleration by the field is negligible compared to the acceleration on anode impact. So the animation above shows approximately what happens in the initial rest frame of the electron during anode impact (anode comes from the right and hits the electron). In reality, due to the acceleration by the field, the lines would be slightly bend initially. But the X-ray is the sharp distortion of the filed lines, emitted on impact.

    Here more nice diagrams:
    http://physics.tamuk.edu/~suson/html/4323/photons.html#Electromagnetic Radiation from Charge
    Last edited by a moderator: Apr 26, 2017
  6. Aug 23, 2011 #5
    WOW! Thanks guys, youre awesomne. All my questions were answered!
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