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.

In an optical fiber, a ray that is refracted from the Core into the cladding. Specifically a ray having direction such that the forumula (shown) 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.