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, φ(r ) is the azimuthal angle of projection of the ray at r on the transverse plane, θ(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.
Source: from Federal Standard 1037C