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If you feed light into the end of a thin glass or transparent plastic fibre, it will be constrained to the fibre by total internal reflection and emerge at the far end, even if the fibre is many miles long. Furthermore, with high quality fibre the losses can be much less than with an electrical cable, and light can carry vastly more data than an electrical signal. Fibre optics is the application of this principle, together with the ancilliary lasers or LEDs for generating the light and the detectors for receiving it at the far end.