No matter how well the glass in a fiber optic cable is made, trace amounts of impurities always are present. These impurities scatter the light traveling through the fiber in all directions, and a small amount ends up going back toward the source.
This scattering of light is a major source of attenuation in a fiber optic cable, and it is desirable to have as little of it as possible. However, it is nearly impossible to eliminate it completely.
On an Optical Time Domain Reflectometer, "normal" backscatter is displayed as a gradually decreasing curve. This is due to the OTDR sending a pulse of light into the fiber and detecting the amount of light which scatters back to it over time. As the pulse moves down the fiber, some of its light is scattered back to the OTDR. The farther the pulse goes from the OTDR, the weaker it gets, so there is less light to be scattered back. Also, the light scattered back toward the OTDR is attenuated in direct proportion to the distance it is from the instrument. Therefore, the amount of backscatter seen by the OTDR is considerably less at 1000 feet than at 100 feet.