Optical fibers are hair-thin strands of glass that can guide light.
The fiber has an inner core surrounded by an outer cladding. Both the core and the cladding are usually made of silica glass, but the core has germanium doped into it to increase its refractive index. The fiber is encased in a plastic coating to protect it. Figure 2 shows how each small bump in the core's refractive index can reflect a little light. These partial reflections can constructively interfere to make a strong back-reflected wave if their spacing is half the wavelength of the incident light.