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Hard X-rays of wavelengths of about an angstrom are very different than regular lights in a way that they can’t be reflected or refracted, which means their refractive index is always close to 1 regardless of media. The reason is that the frequency of X-rays is so high that electrons can’t “catch up with” them.

So my question is at which wavelength does this property emerge? Or in another word, what’s the maximum frequency electrons can oscillate? The wavelength of hard X-rays is at least 3 orders of magnitude shorter than visible lights. The problem is that photons between VUV (~120 nm) and soft X-rays (~1 nm) are extremely easily absorbed by whatever materials, which makes it difficult to determine their refractive index.

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    $\begingroup$ Technically you are not asking about the properties of electromagnetic fields/photons, but you are asking about the properties of optical materials and I am not sure even that is a good question because hard x-ray mirrors (79keV and 122keV) have been made with layered geometries, so it's not "just" the electrons if I am not mistaken. Geometry plays a big role in the manufacturing of x-ray mirrors. $\endgroup$
    – FlatterMann
    Commented Jun 26 at 21:51
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    $\begingroup$ If you mean grazing incidence mirrors, it didn’t contradict my claims which stated that the refractive index of hard X-rays is close to, not equal to 1. Because of the tiny difference in the refractive index, total reflection can occur at the interface when the incidence angle is sufficiently small. $\endgroup$
    – 哲煜黄
    Commented Jun 26 at 21:58
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    $\begingroup$ As far as I know, the refractive index of hard X-rays is always close to 1 which is independent of materials. So there should be a more general mechanism. $\endgroup$
    – 哲煜黄
    Commented Jun 26 at 22:00
  • $\begingroup$ Not as far as the interaction of light with electrons is concerned. We simply can't find materials with higher electron density at normal conditions than some of the high-Z metals. That's pretty much a function of solid state physics but not a property of light, though, and even then we can enhance the existing effect geometrically. $\endgroup$
    – FlatterMann
    Commented Jun 26 at 22:08

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The reason that x-ray frequencies are so high (wavelengths are so short) is because the energy level transitions that give rise to them in solids are of order ~1 kiloelectron volt to ~20 keV. Note that no electron can ever "catch up" with a photon because electrons cannot travel at the speed of light.

The stopping power of a material to x-rays scales as the Z-number of the nucleus. This means that when struck by an x-ray, the material just lets the X-ray penetrate it and scatter around inside it i.e., the x-ray behaves more like a bullet and less like a wave- except at grazing angles of incidence.

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