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Optical fibers are flexible fibers with properties that allow them to channel the movement of photons of light along the fiber's length. A fiber-optic light source emits light that has been sent down an optical fiber. Fiber-optic light sources are used in several different applications, both practical and decorative.
The optical fiber in every fiber-optic light source is a waveguide, meaning that it can guide the movement of photons of light. Light's speed depends on the medium through which it moves. When light crosses the boundary between two media with differing speeds of light, the change in speed causes a change in direction, called refraction. This is the cause of many everyday phenomena, such as the way objects immersed in water appear to be bent when viewed from outside the water. If light reaches the boundary between a medium where it is slower and a medium where it is faster and strikes at a sufficiently large angle, the refraction is so extreme that the light is reflected off of the border between the two and remains in the first medium. This is known as total internal reflection.
An optical fiber is designed with two components, an inner core and an outer cladding, to take advantage of this. Both parts of the fiber are usually silicon dioxide–doped with small amounts of other substances, called dopants. Photons of light are sent into one end of the fiber at a suitable angle to cause total internal reflection within the core, which has a different chemical composition from the cladding due to differences in the dopants each part contains that makes light move through it at a slightly different rate. This channels the photons down the core in the direction of the fiber, which is somewhat flexible and so can be used to guide the concentrated light's direction.
Due to its flexibility, a fiber-optic light source is often used to light enclosed areas that do not have any direct line of sight to an external light source. This makes them useful in applications such as medicine. Some buildings incorporate optical fibers as light pipes or light tubes, which channel sunlight collected from the exterior of the building to provide lighting to locations in the interior. A fiber-optic light source with strands of optical fiber that are designed to intentionally allow significant amounts of light to leak through their cladding and out of the fiber are also used decoratively. This is common in Christmas decorations and can also be incorporated in things such as store displays, clothing, and decorative lights.