In an optical fiber , a ray that is confined primarily to the core . Note: A guided ray satisfies the relation given by
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, n r is the refractive index at the radial distance r from the fiber axis, and n a 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.