Editing Refractive index (section)

===X-ray and extreme UV===
For [[X-ray]] and [[extreme ultraviolet]] radiation the complex refractive index deviates only slightly from unity and usually has a real part smaller than 1. It is therefore normally written as {{math|{{uu|''n''}} {{=}} 1 − ''δ'' + ''iβ''}} (or {{math|{{uu|''n''}} {{=}} 1 − ''δ'' − ''iβ''}} with the alternative convention mentioned above).<ref name=Attwood/> Far above the atomic resonance frequency delta can be given by
<math display="block"> \delta = \frac{r_0 \lambda^2 n_\mathrm{e}}{2 \pi} </math>
where {{math|''r''{{sub|0}}}} is the [[classical electron radius]], {{mvar|λ}} is the X-ray wavelength, and {{math|''n''{{sub|e}}}} is the electron density. One may assume the electron density is simply the number of electrons per atom {{mvar|Z}} multiplied by the atomic density, but more accurate calculation of the refractive index requires replacing {{mvar|Z}} with the complex [[atomic form factor]] {{nowrap|<math> f = Z + f' + i f'' </math>.}} It follows that
<math display="block">\begin{align}
\delta &= \frac{r_0 \lambda^2}{2 \pi} (Z + f') n_\text{atom} \\
\beta  &= \frac{r_0 \lambda^2}{2 \pi} f'' n_\text{atom}
\end{align}</math>
with {{mvar|δ}} and {{mvar|β}} typically of the order of {{val|e=−5}} and {{val|e=−6}}.