Electrons emitted with negligible speed from an electron gun are accelerated through a potential difference V along the x-axis. These electrons emerge from a narrow hole into a uniform magnetic field B directed along this
axis. However, some of the electrons emerging from the
hole make slightly divergent angles as shown in figure
(34-E18). Show that these paraxial electrons are
refocussed on the x-axis at a distance
81?mV
eB?