The diffracted beam
When the electron beam passes through the thin crystalline sample, it is diffracted by the atomic planes in the sample when the Bragg condition is satisfied. These waves interact constructively and are brought to focus at the back focal plane of the objective lens (see Planes) to form the diffraction pattern.
Unscattered electrons continue through to O to produce a central spot. The beam diffracted by angle 2ΘB produces a spot, marked G. The distance between a diffracted (G) and transmitted (O) spot is inversely proportional to the corresponding lattice spacing in the sample.
The beam deflection angle and electron beam wavelength are important.
Bragg's law describes the interaction:
λ = 2d sin ΘB
This equation can be used as long as the wavelength is less than the crystal interplanar spacing (d). This works for a TEM where the accelerated electron beam describes a wavelength of a few pm. This means for most crystalline materials that the Bragg angle is much less than 1°.
The Camera length (projection distance) is also important to know in order to calculate details about the sample. It can be set when photographing a diffraction pattern.