Optical fibers transmit data in the form of light or optical signals. They are made of highly pure glass, so free of impurities that they can transmit 95.5% of a light signal over a distance of one kilometer. This means that, theoretically, one would still be able to clearly view the scenery outside through a kilometer-thick window made of such glass. When you consider that about half of any light passing through a window a few centimeters thick made of ordinary glass would be blocked, then you can appreciate just how transparent the glass used in optical fibers is. It is this very high transparency that enables optical fibers to transmit optical signals over long distances without attenuation.