ABSTRACT
The Bragg law is a necessary condition for diffraction to occur, but it is not a sufficient condition. No diffraction signal may be observed in the direction predicted by the Bragg law. If the Bragg law is not satisfied, diffraction does not take place. The atoms of a crystal are periodically arranged with a repeat unit called the unit cell. The fact that no diffraction signal is observed even under the Bragg condition means that the waves scattered by all the atoms within the unit cell are completely out of phase with one another. The phase relationship between the waves scattered by the individual atoms of a unit cell is a dominant factor affecting the diffraction intensity. The diffraction pattern of a crystal is centrosymmetric irrespective of whether the crystal itself has a center of symmetry or not.
