# Is the index of refraction dependent on the wavelength of light?

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 $n=n(\mathbf{k}, \omega)$.