A crystal can be described as a lattice of atoms, which in turn this leads to the reciprocal lattice. With electrons, neutrons or x-rays there is diffraction by the atoms, and if there is an incident plane wave  with a wavevector , there will be outgoing wavevectors  and  as shown in the diagram after the wave has been diffracted by the atoms.
The energy of the waves (electron, neutron or x-ray) depends upon the magnitude of the wavevector, so if there is no change in energy (elastic scattering) these have the same magnitude, that is they must all lie on the Ewald sphere. In the Figure the red dot is the origin for the wavevectors, the black spots are reciprocal lattice points (vectors) and shown in blue are three wavevectors. For the wavevector  the corresponding reciprocal lattice point  lies on the Ewald sphere, which is the condition for Bragg diffraction. For  the corresponding reciprocal lattice point  is off the Ewald sphere, so  where  is called the excitation error. The amplitude and also intensity of diffraction into the wavevector  depends upon the Fourier transform of the shape of the sample, the excitation error , the structure factor for the relevant reciprocal lattice vector, and also whether the scattering is weak or strong. For neutrons and x-rays the scattering is generally weak so there is mainly Bragg diffraction, but it is much stronger for electron diffraction.