Friedel's law, named after Georges Friedel, is a property of Fourier transforms of real functions.

Given a real function ${\displaystyle f(x)}$, its Fourier transform

has the following properties.

where ${\displaystyle F^{*}}$ is the complex conjugate of ${\displaystyle F}$.

Centrosymmetric points ${\displaystyle (k,-k)}$ are called Friedel's pairs.

The squared amplitude (${\displaystyle |F|^{2}}$) is centrosymmetric:

The phase ${\displaystyle \phi }$ of ${\displaystyle F}$ is antisymmetric:

Friedel's law is used in X-ray diffraction, crystallography and scattering from real potentials within the Born approximation. Note that a twin operation (a.k.a. Opération de maclage) is equivalent to an inversion centre and the intensities from the individual reflections are equal under Friedel's law.