1. A ray that undergoes a change of velocity, or in the general case, both velocity and direction, as a result of interaction with the material medium in which it travels. 2. In an optical fiber , a ray that is refracted from the core into the cladding . Specifically a ray having direction such that
where r is the radial distance from the fiber axis , ( r ) is the azimuthal angle of projection of the ray at r on the transverse plane, ( r ) is the angle the ray makes with the fiber axis, n ( r ) is the refractive index at r , n ( a ) is the refractive index at the core radius, a . Refracted rays correspond to radiation modes in the terminology of mode descriptors.