Ewald's sphere is a geometric construct used in X-ray crystallography which neatly demonstrates the relationship between
- (a) the wavelength of the incident beams of light,
- (b) the angle of diffraction for a given reflection,
- (c) the unit cell and reciprocal unit cell of the crystal, and
- (d) the distance between the crystal and the film.
It was conceived by Paul Peter Ewald, the German physicist and crystallographer.
Ewald's sphere can be used to find the maximum resolution available for a given wavelength of light and the unit cell dimensions. It is often simplified to the two-dimensional "Ewald's circle" model.
Ewald's sphere is also known as the Ewald sphere.