for: diffraction of monochromatic electromagnetic radiation of wavelength
from: parallel planes of atoms separated by a distance 
in an orderly array of atoms (such as a crystal)
determines: the values of the angle 
(measured between the incident ray and the plane of atoms) at which local maxima of intensity are formed in the diffraction pattern by constructive interference of the reflected rays from adjacent planes of atoms. [P7.1]
is normally expressed: as 
where 
is an integer. [P7.1]
Copyright 1997, The Open University