= Magnetic-slab transmission threshold
{title2=$mv/(|q|B)>b$}
A nonrelativistic charged particle entering a uniform magnetic region normally at one plane follows a circle of radius $r=mv/(|q|B)$. If the field extends through a normal thickness $b$, the particle enters the strict region beyond the far plane only for $r>b$. Equality gives tangency, not crossing. For transmission, its deflection angle obeys $\sin\theta=b/r$. The result assumes a sharp field boundary, fixed speed in the magnetic region and no electric field there.
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