The theory of photon interference X-ray absorption fine structure (
XAFS) is described. Due to coherent X-ray scattering from atoms, a spatial variation of the X-ray intensity is produced inside the sample. The intensity at the X-ray absorbing atom changes according to the incident energy. Thus
XAFS in extended absorption spectra is produced. It extends in a wide energy range over absorption edges. For powders the
XAFS formula has equivalent form as the EXAFS formula, and the Fourier transform provides distances of neighboring atoms from the absorbing atom. Due to a long mean free path of the photon,
XAFS for powders contains sharp structures. They are explained as a crystal grain orientation averaging of the X-ray standing wave effect.