In telecommunications, reflection loss occurs on a line which results in part of the energy being reflected back to the source. This can occur: 1. * At a discontinuity or impedance mismatch, e.g. in a transmission line, the ratio of the incident power to the reflected power. Reflection loss is usually expressed in dB. 2. * In an optical fiber, the loss that takes place at any discontinuity of refractive index, especially at an air-glass interface such as a fiber endface, at which a fraction of the optical signal is reflected back toward the source. This reflection phenomenon is also called "Fresnel reflection loss," or simply "Fresnel loss."