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. (188) Synonyms bound ray, trapped ray.