Science Puzzle
How Can Light Travel Round Corners in a Glass Fibre?
Light travels in straight lines. Yet shine a laser into one end of a thin glass fibre, bend the fibre round a corner, and the light comes out of the far end. Cables full of these fibres carry the internet under the oceans, round countless bends, for thousands of kilometres.
How does the light follow the fibre round the bend?
The Answer
The light bounces along inside the fibre. When light inside glass meets the edge at a shallow, glancing angle, none of it escapes: it's all reflected back in. So the light zigzags along the fibre, round any gentle bend, without ever getting out.
Light slows down in glass, and when it crosses from glass into something where it travels faster, it bends away from the boundary. Hit the boundary at a steep angle and most of it passes out. Hit it at a shallow enough angle, less than about 48 degrees from the surface for ordinary glass and air, and the light can't get out at all. Every bit of it reflects, better than off any mirror. This is called total internal reflection, and you can see it from under the water in a swimming pool, where the surface looks silvery when you look up at a slant.
A real optical fibre is a glass core wrapped in a second layer of glass that light travels slightly faster in, which keeps the reflections perfect even when the fibre is touched or dirty. The glass is astonishingly clear: light can travel about 15 kilometres before it fades to half its strength. In 2009 Charles Kao shared a Nobel Prize for showing, back in the 1960s, that pure enough glass could carry light over such distances.
The principle: Total internal reflection. Light inside glass that meets the surface at a shallow enough angle is reflected completely back in, so a glass fibre can guide light round bends.