ABSTRACT
This chapter introduces a very useful concept that links the reciprocal lattice to Bragg’s Law. This is the reflecting sphere, otherwise known as the Ewald sphere because it was first described by P. P. Ewald in 1919 as an aid to the interpretation of diffraction patterns. The intensity is at a maximum at the centre of the streak, i.e. when the ‘spot’ is exactly on the reflecting sphere. Orientation relationships between phases are usually described in terms of a pair of parallel directions in a pair of parallel planes. If a diffraction pattern consists of the superimposed patterns from more than one phase, the technique of dark-field imaging may be used to determine which spots originate from which phases.
