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If, in a crystal structure, atoms lie in the neighbourhood of a set of planes
*H*, as indicated in Fig. 1a, then reflection by planes *H* is strong and hence
the intensity *I*_{H} is large. Of course, the converse is also true: if one
observes a large intensity *I*_{H}, then the atoms lie near planes as indicated in
Fig. 1a. This statement follows also from the structure-factor expression:

A large *F*_{H} will be found if (*hx*_{j} + *ky*_{j} + *lz*_{j}) mod 1 is approximately
constant for all *j*; or, in other words, if all atoms lie near one of the
planes *H*. The phase depends on the value of the constant and changes
with the origin.

Conversely, a structure-factor magnitude |*F*_{H}| is small, if the atoms are
randomly distributed with respect to the planes *H*, as shown in Fig. 1b.

The electron density can be thought of as a superposition of density waves
parallel to lattice planes, the amplitudes of which are the |*F*_{H}|-values, the
relative phases being given by the -values. We will see later that
these density waves afford a physical picture of the phase relationships used in
Direct Methods.

**Copyright © 1984, 1998 International Union of
Crystallography**