guided ray
In an optical fiber, a ray that is confined primarily to the core.
Note: A guided ray satisfies the relation given by ,f21 where 
r is the angle the ray makes with the fiber axis, r is the radial position, i.e., radial distance, of the ray from the fiber axis, nr is the refractive index at the radial distance r from the fiber axis, and na is the refractive index at the core radius, a, i.e., at the core-cladding interface. Guided rays correspond to bound modes, i.e., guided modes, in terms of modes rather than rays. Synonyms bound ray, trapped ray.