The optical reciprocity theorem, or principle of Helmholtz reciprocity, generally holds true for elastically scattered electrons in an absorbing medium, as is often the case under standard TEM operating conditions. The theorem states that the wave amplitude at some point B as a result of electron point source A would be the same as the amplitude at A due to an equivalent point source placed at B. Simply stated, the wave function for electrons focused through any series of optical components that includes only scalar (i.e. not magnetic) fields will be exactly equivalent if the electron source and observation point are reversed.