branch of physics that is concerned with beams of electrons, their deflection and focusing by electric and magnetic fields, their interference when crossing each other, and their diffraction or bending when passing very near matter or through the spacings in its submicroscopic structure. Electron optics is based on the wave properties of electrons, which, according to quantum theory, can be treated either as particles or as waves. The wave behaviour was predicted and then experimentally established in the 1920s. Beams of electrons exhibit behaviour similar to those of light and X rays, and all these are subject to the same mathematical descriptions. One of the applications of electron optics is the electron microscope.