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This diagram describes the operation of an electron lens
in the electron microscope.


Looking inside the lens, we see that the soft iron polepieces have a gap
within them, to form a radially symmetric magnetic field in the central part
of the lens.


Magnetic field lines emanate from the gap in the polepieces. The
magnetic field is shielded elsewhere.


Since it is charged, an electron entering the lens experiences a Lorentz
force perpendicular to its motion and the magnetic field.


If the electron is moving slightly off axis it will have a component of
velocity radially outward. This means that it experiences a horizontal
Lorentz force normal to this (red arrow).


The electron hence moves
along an inwardly spiralling helix as it moves through the magnetic field.


In this way, the cone of electrons is focussed towards a point.


In this way, the cone of electrons is focussed towards a point.