Fresnel Reflection
When light enters the core of a fiber and strikes the cladding at an angle less than the critical angle then most of the light energy is refracted into the cladding and is lost (as is desired). A very small amount of light will be reflected back into the core. This reflected light is referred to as 'Fresnel Reflected' light. It is generally less than 4% of the total incident light energy and therefore generally not powerful enough to carry a spurious signal to the other end of the fiber. This is illustrated in Figure 3.
Figure 3. Fresnel
Reflection