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I just read in some class notes for a crystallography class that there are no refractive lenses for X-rays because the index of refraction of most materials is close to 1. Is the index of refraction dependent on the wavelength of the light then? Like, a material has different indices for different types of electromagnetic waves?

Thank you.

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It is possible that refraction index varies according to the wavelength of the light. It's called temporal dispersion. In fact, at optical frequencies, metals are dispersive, which means the refractive index changes with different wavelength. Moreover, the material parameters may have different response with different incident angles, say, the wave vector $\mathbf{k}$ is different. In this case, we call it spatial dispersive. As you go to subnanometer scheme, nonlocal effects appear, say $n=n(\mathbf{k}, \omega)$.

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Yes the refractive index of a material depends on wavelength. This phenomenon is called dispersion. In fact, it's precisely this phenomenon, for example, that causes prisms to separate white light into its constituent colors. Since each color refracts a different amount when they enter the material, the colors separate.

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