Graphite films have a polyscystalline structure. In other words, graphite films are composed of very many individual crystals that are randomly arranged in space. Because of this structure, for every electron of wavelength λ there is a crystal spacing that fulfills the Bragg condition. In the illustration, electrons with various velocities - and therefore various wavelengths as well (for example \(\lambda_1\), \(\lambda_2\) und \(\lambda_3\)) - fall upon the graphite film. For every electron wavelength, there is a corresponding crystal structure that fulfills the Bragg condition.
In this experiment, electrons are fired at the graphite film; all electrons have approximately the same velocity because they are fired from the same electron beam gun. Similarly, all electrons have the same wavelength. All electrons reflect from the appropriately oriented crystal and constructively interfere to produce the interference maximua. The interference maximum appears on the screen as a brighter rings. The angle between the incident and reflected electron beams which satisfies the Bragg condition is twice as large as the angle of incidence \(\theta\) for which the Bragg condition is satisfied - that is, \(2\theta\) (see illustration).
The maxima are circular because all the crystals are oriented randomly in space. Some crystals are oriented such that they produce the "top" of the interference maximum in relation to the beam direction, while others are oriented to produce the left, right, bottom, or other parts of the interference maximum. The result is a circular ring at the angle \(2\theta\)