An optical fibre traps light so it cannot leak out of the sides

1. A fibre is built as two layers: an inner core that carries the light, wrapped in an outer layer of cladding.
2. The cladding is chosen to have a lower refractive index than the core, so light in the core is always heading towards a less dense material at the boundary.
3. The core is very thin, often thinner than a human hair, so light entering it can only ever strike the sides at a glancing angle.
4. That glancing angle, measured from the normal to the side wall, is larger than the critical angle, so the light is totally internally reflected rather than escaping.
5. The ray therefore zig-zags along the fibre, bouncing thousands of times a metre, and because the reflection is total, almost none of the light is lost at each bounce.
6. Bending the fibre gently just changes the angle slightly at each bounce, so the light follows the curve and the fibre can be threaded round corners.
7. Bend it too sharply and the angle drops below the critical angle at the outside of the bend, so light refracts out and the signal weakens.

In telecommunications the message is sent as pulses of light

1. A phone call, a video stream or a web page starts out as an electrical signal at the sending end.
2. A laser or light-emitting diode converts it into extremely rapid on-and-off pulses of light, one pulse for each binary digit.
3. Those pulses travel down the fibre by total internal reflection, at close to the speed of light in glass.
4. At the far end a detector turns the light pulses back into an electrical signal, which the phone or computer decodes.
5. Because light has such a high frequency, pulses can be sent billions of times a second, and many separate signals can share the same fibre at once.
6. Over very long routes the pulses do fade, so repeaters are spaced along the cable to boost them back to full strength.

Why fibres replaced copper wires for carrying information

1. They carry far more information per second, because light pulses can be packed much closer together than electrical ones.
2. The signal fades much less over distance, so repeaters can be tens of kilometres apart instead of every few kilometres.
3. Light is unaffected by electrical interference from motors, power lines or lightning, so the signal stays clean.
4. Fibres are thinner and lighter than copper cables of the same capacity, so far more of them fit into an existing underground duct.
5. They are more secure, because a fibre gives off no signal that can be picked up from outside without cutting into it.
6. They also carry no current, so there is no risk of a short circuit or a spark, which matters in a fuel depot or a chemical plant.

Fibres are also used to see inside things

1. An endoscope uses two bundles of fibres to let a doctor look inside a patient without surgery.
   1. One bundle carries bright light down to illuminate the inside of the body.
   2. The other carries the image back out, with the fibres kept in the same order at both ends so the picture is not scrambled.
2. The same idea lets an engineer inspect the inside of an aircraft engine or a wall cavity through a small hole.
3. Every one of these uses rests on the same physics: the light is trapped because it meets the side of the fibre above the critical angle.