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Why are X-rays scattered by matter?

  1. Oct 27, 2013 #1
    Why are X-rays scattered by matter..???

    1 Why are X-rays scattered by atoms in a material, though their frequencies are too high and for most of the atoms,electron clouds cannot respond to such a high frequency(that is before the electron cloud can reorient in the direction of electric field of X-ray, the field reverses,and consequently ,electrons do not respond to X-rays).

    2 And even if the cloud does oscillate,it oscillates very very weakly and 180 degrees out of phase with the driving force,as the frequency of the driving force(electric field),in case of x-rays is much higher than resonant frequency of most atoms.

    3 So,how is Thomson Scattering,in which electron cloud moves in phase with incident light and scatter light, possible in case of X-rays...?????

    I am attaching hereby a material for proof of the point 2 which I have mentioned here. In that material refer to the diagram on page 64 and also the diagram on page 65

    https://www.physicsforums.com/showthread.php?t=718499 . In this link, open the file attached in post no. 13

  2. jcsd
  3. Oct 27, 2013 #2

    Simon Bridge

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    x-rays can interact with matter by photo-ionization and by Compton scattering.
  4. Oct 27, 2013 #3
    Yes, I do agree with u Simon Bridge.

    But don't X- rays interact with matter by Thomson Scattering.If yes, then how do they,is my question since necessary condition for Thomson scattering is that electrons should move in phase with X-rays.

    But theoritically, it seems impossible as most of matter have resonance frequencies much below X-rays(ω>>>ω0) case. [Refer to the diagram on page no. 64 of the material I suggested]

    Hence forth, X rays cannot make electrons oscillate in phase with their electric field,instead electrons oscillate 180 degrees out of phase with x ray electric field.

    So, how is X ray thomson scattering possible.....????

  5. Oct 27, 2013 #4

    Simon Bridge

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  6. Oct 27, 2013 #5
    So, Mr. Simon Bridge, if the electrons behave as if they are free electrons under the influence of X-rays then, they must oscillate 180 degrees out of phase with the driving force(electric field force). This is one of the fundamental principles of physics.

    And if the electrons oscillate 180 degrees out of phase then the phase out the Thomson Scattered X-rays must also be 180 degrees out of phase.

    Am I right...????? Do you agree with me on this point...??????


  7. Oct 27, 2013 #6


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    Where exactly did you get this?

    In an X-ray free-electron laser, the electron bunch oscillates through a wiggler/undulator at the same frequency as the X-ray that it produces So already such electrons CAN respond to such high frequency.

    Since this is your starting point, unless this is clarified and verified first, the whole subsequent premise might be wrong.

  8. Oct 27, 2013 #7
    I got that from a post in a thread belonging to this site(PhysicsForums) only. The link is attached below. Have a look at the post number 6. In that it is clearly mentioned that, x-rays penetrate a human body easily because they are high enough frequency that most dielectric effects have relaxed out - meaning they can't respond faster enough to the incoming wave.


    And we must also keep in mind that as much of the bandwidth of X-rays have frequencies much higher than resonance frequencies(ω>>>ω0) or if I explain this phenomenon in modern terms,most of the frequencies lying in X rays bandwidth have energy much more than that of binding energy of electrons of atoms, for most substances.

    So even if they(electrons) do respond,they will respond 180 degrees out of phase with incident X-ray beam and the amplitude of electron oscillations decrease as per the term 1/ω2.

    Here note that there may be some inner shell electrons in an atom which have their resonance frequencies less than that of X-ray beam(ω<<<ω0),and so they might oscillate in phase with the beam.

    I am not making up stories myself. Iam not saying that I am perfect or I know each and everything,but I am not also totally wrong.

    I am just asking whether the phase of the Thomson scattered beam depends on the binding of the electrons in the atom or not....???? That is if the binding is loose and electrons of atoms behave as free electrons, will not the Thomson scattered beam 180 degrees out of phase with the electric field(driving force) of incident X-ray beam ....???
  9. Oct 27, 2013 #8


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    This is why we require that people cite their sources! You've read something and completely misunderstood what you've read! You did exactly what I described in #3 in here:

    https://www.physicsforums.com/blog.php?b=2703 [Broken]

    There's nothing here about electron clouds not being able to respond to X-rays. You seem to think that when EM radiation enters a material, only electrons are solely responsible for any and all interactions with that EM radiation. This is false!

    The dielectric constant of a material, a property that can severely affect light's behavior with the material, is a function of the whole material, including the ions, and how those ions are arranged! After all, look at how different graphite and diamonds are, and yet, they are both made of predominantly carbon atoms!

    The starting premise of your question is faulty. Electrons CAN respond to x-rays. It is why the rest of your question doesn't make much sense.

    Last edited by a moderator: May 6, 2017
  10. Oct 27, 2013 #9
    Ok Mr. ZapperZ, I lose,you win here, in this point. I give up here. This Graphite and Diamond example has reminded me of a physics FAQ posted by you,namely, "Do Photons Move Slower In A Solid Medium".I am now getting somewhat what you want to tell.

    But atleast just help me to confirm whether I am thinking right on the following two points mentioned below -

    1) If it happens that for X-rays, ω>>>ω0 case is true for some electron in some material then, that electron will respond 180 degree out of phase with the driving force(electric field) of the incident X-ray beam and the amplitude of electron oscillation decreases as per the term 1/ω2.

    2) If for some electron in some material, ω>>>ω0 for a particular frequency ω of X-ray then that particular electron almost behaves as a free electron and hence forth that particular electron oscillates 180 degree out of phase with the driving force(electric field) and so consequently the Thomson scattered X-ray beam is 180 degrees out of phase relative to the incident X-ray beam.
  11. Oct 27, 2013 #10

    Simon Bridge

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    Free electrons do not oscillate with photons - the photon and the electron scatter off each other.
  12. Oct 28, 2013 #11
    Yes, that is if you consider particle nature of light. But if you consider light to be a pure classical electromagnetic wave, and free electrons(in materials) to be particles, they do oscillate under the influence of light and a vibrating electron is an emitter of light in all directions. The intensity of the re-emitted light is directly proportional to the square of amplitude of electron oscillation and also to the fourth power of angular frequency(ω) of vibrating charge(electron).

    None of our concepts are wrong. You are thinking the quantum way and I am thinking the classical way. And to understand nature both the ways of thinking are equally important. This is what I believe.
  13. Oct 28, 2013 #12

    Simon Bridge

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    Except that we know that the classical model only provides approximations - which is why we use quantum mechanics. This is particularly true for the higher energy limits for light.

    As you explore these energies you will find the classical models more and more unwieldy.
  14. Oct 29, 2013 #13

    Jano L.

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    EM radiation, of how great soever high frequency, always makes the charged particles to radiate their own waves.

    Free charged particle without self-action always oscillates with coordinate having 180° shift with respect to the external wave. That is one way to describe the Thomson scattering of X-rays off nitrogen molecules; the frequency of X-rays is so high that the electrons can be considered as free.

    2) Basically yes, except for higher frequencies new phenomenon begins to appear, Compton scattering: the matter reradiates at different frequencies, depending on the angle of scattering, which requires more involved explanation.

    3) In the Thomson scattering, the electron cloud does not move in phase with incident electric field. That can happen only for light with frequency far from the resonance frequencies of the atoms/molecules and below the highest resonance frequencies.

    For example, the phase match happens for visible light in the air: the corresponding EM wave has frequency that is not absorbed much by the molecules and is below their max. UV resonance frequencies. That's why the air is largely transparent and the scattering is weak; it is not the Thomson scattering, but the Rayleigh scattering.
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