An optical fibre is a solid rod of transparent material, which can be as fine as a strand of human hair, designed to efficiently guide light from one place to another.

The most common raw material for making optical fibres is very pure silica sand. This is used because it is relatively inexpensive and can be formed into a glass so pure that a solid block 35 kilometres thick would be as transparent as a pristine clean windowpane. Ordinary glass would lose the signal beyond recovery within a few metres, as the impurities would scatter the light in the fibre like light lost in fog.