Refraction And Total Internal Reflection P

Somewhere on the floor of the Atlantic there is a cable about the thickness of a garden hose, and inside it are strands of glass finer than a human hair. The words on this screen may well have crossed an ocean inside one of them, as flashes of light bouncing off the inside of the glass tens of thousands of times a second without a single flash escaping. Nothing holds that light in. There is no mirror, no coating, no clever machinery. The light simply cannot get out, and the reason is one of the neatest arguments in the whole of school physics.

It begins somewhere much more ordinary: with the fact that light travels at different speeds through different materials. Slow a wave down as it crosses a surface at an angle and it has no choice but to swing round, which is why a straw in a glass of water looks broken and why a swimming pool is always deeper than it looks. In this lesson you will learn how to put a number on that bending, how to calculate exactly how far a ray turns, why a glass prism splits sunlight into colours, and where the angle lies beyond which light stops escaping altogether and gets trapped. By the end you will be able to work out that angle for any material from a single measurement, and explain how surgeons see inside a living body through a bundle of glass threads.