2. Reciprocal space

Bragg's condition stipulates that the angle of reflection, on an atomic plane, is simply defined by the wavelength of the radiation used and by the inter-reticular distance, i.e. the distance between the atomic planes. But even if this condition is met, there may still be no reflection allowed. To determine when reflections are permitted or forbidden, Ewald introduced the notion of reciprocal lattice in 1921 .