As the light wave traveling in the fiber reaches the boundary between the cladding and core, it bends back into the core. The bouncing and bending of the light causes it to travel fast thus light and information are transmitted fast.
There are usually two types of optic fibers: single and multi-mode fibers. The single mode fiber features a small core (about 10 micrometers) and a cladding of 100 micrometers in diameter. Since the cable is small, it carries only one light wave over a long distance. Bundles of the single-mode fibers are heavily used in undersea cables and long distance telephone lines.