= Magnetic electron lens
= Magnetic electron lenses
{synonym}
A <magnetic electron lens> uses a <solenoid> around the beam axis; iron pole pieces concentrate its <magnetic field>.<The Development of the Electron Microscope and of Electron Microscopy by Ernst Ruska>[Ruska, §§C and E, Fig. 4: iron enclosure and pole-piece lens]{ref} A strictly axial trajectory in a perfectly uniform axial <magnetic field> experiences no <Lorentz force>, since $\mathbf v\times\mathbf B=0$. Near the entrance, a radial field component turns an advancing <Electron> azimuthally; the resulting sideways <velocity> in the axial <magnetic field> also produces a radial <Lorentz force>. This couples image rotation and focusing. Real trajectories bend smoothly, whereas a <thin lens> diagram compresses that bending into one effective plane.
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