Constructive interference, among waves

In a collection of atoms containing many electrons, the scattered radiation collected at a distant point P shown in Figure 3 (drawn after Ref. 6) will depend upon the phase relationship among the waves scattered from each atom. A large intensity will be measured at P provided that there is constructive interference among the scattraed waves emitted by the atoms. On the other hand, if crests from some scattered waves overlap... 

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