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, a 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.

Crystallography