Ewald's sphere

The Ewald sphere is a geometric construction used in electron, neutron, and x-ray diffraction which shows the relationship between:

• the wavevector of the incident and diffracted beams,
• the diffraction angle for a given reflection,
• the reciprocal lattice of the crystal.

It was conceived by Paul Peter Ewald, a German physicist and crystallographer. Ewald himself spoke of the sphere of reflection. It is often simplified to the two-dimensional "Ewald's circle" model or may be referred to as the Ewald sphere.