= Optical fiber
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= Optical fibre
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An <optical fiber> is a thin, flexible <electromagnetic waveguide> that carries <light> along its length. In a conventional <step-index optical fiber>, an <optical fiber core> with <refractive index> $n_1$ is surrounded by an <optical fiber cladding> with lower <refractive index> $n_2$. Suitable <light rays> remain confined by <total internal reflection> at this interface. The cladding is an optical part of the guide, distinct from its protective coating.<Introduction to Optical Fibers, dB, Attenuation and Measurements by Cisco>[Introduction to Optical Fibers, dB, Attenuation and Measurements, Optical Fiber Structure, Fig. 1]{ref}
The useful surprise is that a transparent strand can deliver <light> around a route that a free beam would leave. This makes <optical fibers> useful for communications and for carrying collected <light> to a <fiber-fed spectrograph>. Guidance, successful launch, and surviving <optical power> are separate questions: the <optical fiber acceptance angle> constrains launch geometry, while <optical fiber attenuation> describes loss along the route.
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