= Step-index optical fiber
A <step-index optical fiber> has a nearly uniform <refractive index> $n_1$ throughout its <optical fiber core>, with an abrupt decrease to $n_2<n_1$ in its <optical fiber cladding>. In a straight, ideal guide, <meridional rays> stay in a plane containing the <optical axis> and trace a zigzag through repeated <total internal reflection>. This is a <geometrical optics> approximation; it is particularly useful for a <multimode optical fiber> whose core diameter is large compared with the <wavelength>.
\Image[/optical-fiber-launch-and-interface-angles.png]
{title=Launch angle to the axis and incidence angle to the core–cladding normal}
The upper panel shows an accepted <meridional ray>; the lower panel enlarges the limiting geometry. The <angle of incidence> $i$ at the side wall is measured from its <normal vector>, whereas the internal ray <angle> $\phi$ and external launch <angle> $\theta$ are measured from the <optical axis>. Thus $i+\phi=90^\circ$. The limiting ray is a boundary of the ideal model, rather than a complete <normal mode> solution.
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