Electron
diffraction is a collective scattering phenomenon with
electrons being (nearly elastically) scattered by atoms in a regular array (crystal).
This can be understood in analogy to the Huygens principle for the diffraction of light. The incoming plane electron wave interacts with the atoms, and
secondary waves are generated which interfere with each other.
This occurs either constructively (reinforcement at certain
scattering
angles
generating diffracted
beams) or
destructively
(extinguishing of beams).
As in Xray diffraction (XRD), the scattering event can be described
as a reflection of the beams at planes of atoms (lattice planes).
The Bragg law gives the relation between
interplanar distance d and diffraction angle Θ which is the distance between the reflection and the origin of the reciprocal lattice:
nλ =
2dsinΘ
Each set of parallel lattice planes, which correspond to planes decorated with atoms in the structure, generates a pair of spots in the electron diffraction pattern with the direct beam in their center (see scheme below).
Since
the wavelength λ of the electrons is known, interplanar
distances can be calculated from ED patterns. Furthermore,
information about crystal symmetry can be obtained. Consequently,
electron diffraction represents a valuable tool in crystallography.

Estimate
of scattering angles
λ_{el} =
0.00197 nm (1.97 pm) for 300 kV electrons. A
typical value for the interplanar distance is d = 0.2 nm.
If these values are put in the Bragg law, then the scattering angle is: Θ = 0.28°.
As
a rule, the scattering angles in ED are rather small (e.g., compared to those in XRD): 0 < Θ
< 2.
From
this follows that
(i) the reflecting lattice planes are almost
parallel to the direct
beam (left figure),
(ii)
the incident electron beam is the zone axis of the reflecting sets
of lattice planes (right figure). 