Reflections as Fourier terms Equation

First consider Eq. (5.18) (p in terms of Fs). Each term in this Fourier-series description of p(x,y,z) is a structure factor representing a single X-ray reflection. 

Chapter 5. From Diffraction Data to Electron Density 

The indices hkl of the reflection give the three frequencies necessary to describe the Fourier term as a simple wave in three dimensions. Recall from Chapter 2, Section VI.B, that any periodic function can be approximated by a Fourier series, and that the approximation improves as more terms are added to the series (see Fig. 2.14). The low-frequency terms in Eq. (5.18) determine gross features of the periodic function p(x,y,z), whereas the high-frequency terms improve the approximation by filling in fine details. You can also see in Eq. (5.18) that the low-frequency terms in the Fourier series that describes our desired function p(x,y,z) are given by reflections with low indices, that is, by reflections near the center of the diffraction pattern (Fig. 5.2). 

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