The document discusses optical fibers, which transmit light through total internal reflection. Optical fibers consist of a core and cladding material, with the core having a higher refractive index to allow light to reflect off the core-cladding boundary. Single mode fibers have a small core diameter of around 9 microns while multi-mode fibers have a larger core of around 50 microns. Optical fibers can transmit both analog and digital signals over long distances using transmitters to inject light and receivers to detect the light.