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Confusion about Wave / Particle Behaviour of X-Rays

  1. Sep 4, 2012 #1
    I am a Physics instructor teaching a course on Physics for Medical Radiologists and have a question to further my own understanding of wave / particle behaviour of X-Rays as used in x-ray image production. My background is more in climate / geophysics.

    The textbooks available for this course (written at a very basic Physics level) briefly describe both the wave and particle nature of light and then go on to suggest that X-Rays, as far as medical radiologists are concerned typically behave like particles (ie. should be envisioned as photons, rather than EM waves) and don't exhibit wave like behaviour in this application.

    However, I also know that X-Rays, as used in applications such as crystallography, rely on the radiation's essentially behaving as waves. Other pieces of Physics writing I've read written for the lay person describe high-energy light as behaving in a "particle like fashion" and lower energy light as behaving in a wave light fashion when interacting with matter.

    My round about question is therefore this: Is there a general rule that one can apply in terms of a relationship between the wavelength of light and the nature or characteristic length scales of the matter it is interacting with in terms of when one can state that the light behaves "as a particle" and when it behaves "as a wave" when interacting with matter?

    Secondly, and this part is more directed at instructors, I suppose. Is it reasonably correct to say that, when one is visualizing a beam of x-rays as behaving like particles to think of this beam as being essentially similar to a beam of electrons, made up of discrete chunks of quantized energy?

    I would appreciate any guidance and clarification anyone can offer.

  2. jcsd
  3. Sep 5, 2012 #2
    The whole "high-energy = particle, low-energy = wave" thing is a good rule of thumb for the layperson... as long as it applies. :devil:

    In many scenarios, x-rays take on a predominantly particle nature. One exception, as you mentioned, is crystallography (i.e. x-ray diffraction (XRD)). The reason x-rays take on a predominantly wave nature in crystallography is because of the dimensions involved. Typical XRD wavelengths are around 1.5 angstroms (0.15 nm). Typical lattice spacings in crystals are 5 - 15 angstroms. Clearly, the bones of your hand are much larger than this, and so x-rays take back their particle nature in many/most medical applications. (I believe that medical x-ray wavelengths are around 10 angstroms.)

    A good rule of thumb is this:
    If the characteristic dimension of your structure is on the order of or smaller than the wavelength, then the wave picture prevails. If the characteristic dimension is much larger than the wavelength, then the particle picture prevails.

    Just to give you some further insight, the radio "waves" used in satellite TV are a few centimeters in wavelength. To a first approximation, any meter-wide satellite dish can be analyzed by simple ray tracing. This is a characteristic of particle-like behavior. Funny that you never hear of "radio particles!" :tongue2:
  4. Sep 5, 2012 #3
    Also, to answer your second question: that's the way I think of it.
  5. Sep 5, 2012 #4
    cmos rule of thumb works very well. But in general a quantum mechanical particle never looses it wave-like nature and vice versa.

    In some extreme cases (very small x-ray source very far away) one can get wave-effects even in imaging. This is called phase contrast imaging and is used in high-resolution imaging with synchrotron x-rays.



  6. Sep 5, 2012 #5
    Thanks cmos and M Quack. Those responses really helped.


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