How does a fiber optic cable work?

Over the last 20 years or so, fiber optic lines have taken over and transformed the long distance telephoneindustry. Optical fibers are also a huge part of making the Internetavailable around the world. When fiber replaces copper for long distance calls and Internet traffic, it dramatically lowers costs.     Tounderstand how a fiber optic cable works, imagine an immensely longdrinking straw or flexible plastic pipe. For example, imagine a pipethat is several miles long. Now imagine that the inside surface of thepipe has been coated with a perfect mirror. Now imagine that you arelooking into one end of the pipe. Several miles away at the other end,a friend turns on a flashlight and shines it into the pipe. Because theinterior of the pipe is a perfect mirror, the flashlight's light willreflect off the sides of the pipe (even though the pipe may curve andtwist) and you will see it at the other end. If your friend were toturn the flashlight on and off in a morse code fashion, your friendcould communicate with you through the pipe. That is the essence of afiber optic cable.     Making a cable out of a mirrored tube wouldwork, but it would be bulky and it would also be hard to coat theinterior of the tube with a perfect mirror. A real fiber optic cable istherefore made out of glass. The glass is incredibly pure so that, eventhough it is several miles long, light can still make it through(imagine glass so transparent that a window several miles thick stilllooks clear). The glass is drawn into a very thin strand, with athickness comparable to that of a human hair. The glass strand is thencoated in two layers of plastic. By coating the glass in plastic, you get the equivalent of a mirror around the glass strand. This mirror creates total internal reflection,just like a perfect mirror coating on the inside of a tube does. Youcan experience this sort of reflection with a flashlight and a windowin a dark room. If you direct the flashlight through the window at a 90degree angle, it passes straight through the glass. However, if youshine the flashlight at a very shallow angle (nearly parallel to theglass), the glass will act as a mirror and you will see the beamreflect off the window and hit the wall inside the room. Lighttraveling through the fiber bounces at shallow angles like this andstays completely within the fiber.