= Meridional ray bend threshold
{title2=$R_c=n_2r/(n_1\cos\beta-n_2)$}
For a <light ray> launched on the centreline in the plane of a circular <optical fiber> bend, let $R$ be centreline radius, $r$ core radius and $\beta$ the internal launch <angle> from the tangent. The straight segment's distance from the bend centre is $R\cos\beta$. Thus its outer-wall <angle of incidence> satisfies $\sin i=R\cos\beta/(R+r)$. Equating this to $n_2/n_1$ gives the limiting <total internal reflection> threshold in the title, provided $n_1\cos\beta>n_2$. Radially displaced or nonplanar launches need different geometry; this is not a universal wave-optical loss cutoff.
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