Focusing of an e-beam is done by applying a magnetic field parallel to an optical axis along which the beam is required to travel. The direction of the magnetic field, optical axis, and velocity of the electron is shown. An e-beam, after being accelerated by an applied voltage, does not exactly move in a straight line along the optical axis but moves in a curved path due to the presence of other electromagnetic fields. If an e-beam entering the magnetic field makes an angle with the optical axis, it will be deflected by the field and will become parallel with the optical axis.