In electron imaging, TEM and STEM are considered to be equivalent, so that images obtained in both optic configurations have many analogous characteristics, and sometimes they can be interpreted using the same theory. This analogy is based on the reciprocity theorem, which is stated as: The amplitude of the disturbance at a point P due to radiation from a point Q, which has traversed any system involving elastic-scattering processes only, is the same as the amplitude of the disturbance that would be observed at Q if the point source were placed at P.