How does an optical fiber guide a light signal around a bend?

A light signal can follow a thin glass fiber even when the route curves. The fiber has an inner core surrounded by cladding. These are chosen so that light travels more slowly in the core than in the cladding: the core has a higher refractive index.

At the boundary, light arriving from the higher-index core can be completely reflected back inside. This happens only when its angle from the normal exceeds the critical angle. The normal is an imaginary line perpendicular to the boundary; a large angle from it means the ray meets the boundary at a grazing angle.

For the supplied core index of 1.5 and cladding index of 1.4, the critical angle is about 69 degrees. The illustrated 80-degree ray meets the condition for total internal reflection. A 60-degree ray does not, so some light can pass into the cladding instead of remaining wholly reflected.

A gently curving fiber can keep guided light meeting suitable boundary conditions, carrying encoded changes in light to a receiver. It cannot bend without limit: tight bends can let light escape. Real signals also weaken through absorption and scattering. The local ray diagram explains confinement, not loss-free transmission or a complete wave description of fiber modes.