How Does Optical Fiber Work?

Video Transcript

"Hi I'm Steve Jones and I'm going to show you how an optical fiber works. In England we spell fiber bre and I know in America ber. First of all looking at a more traditional thing this is a solid prism of glass. A prism triangular in this case, 45 degree, 90 prism of glass. If I have a beam of light coming in here through the side it goes into the glass and it is reflected from the inside of the glass and will be reflected out back the way it came. This is used in binoculars to extend the length of the tube so that they can be quite small instead of having to be long like a telescope so in binoculars we see this arrangement already well known this idea of total internal reflection. It works at the angle here between what we call the normal this line and the ray is more than 42 degrees. Well 45 degrees here. If we have an optical fiber we have the same situation. The interesting thing about an optical fiber is that in fact it is solid glass. It is not a tube. People often see a diagram and think it is a tube. It is not. There is no air inside it. It is solid glass. Very often the glass is different in density but the outside from the inside, the glass and the nature of the glass changes slightly and that means that this process isn't quite like total internal reflection but it works in the same way. The light ray comes in and because of the angle at which it hits the side of the fiber is greater than the 42 degrees this means that it is reflected. Nothing comes out of the glass and in fact fibers are also coated but they are not coated to keep the light in they're coated to protect the fiber so you have got continuous reflections down the light fiber and out it comes at the other end. Now can you imagine if you had a thousand or a million of these fibers in a bundle the light going in at this end depending on where it lands in that bundle will come out at the other end each as long as the bundles are in the same order as it comes out, an image at this end will be exactly the same as the image at the other end. In other words you can use this optical fiber to transmit pictures which is what they do when they use one of these scopes that goes inside the body it has an optical fiber and the optical fiber brings back the information. Optical fibers now are used not just for this though. The most common use nowadays is in telephony, telephones and radio and TV. So cable relies on these fibers. The light is directed down them and little pulses of light are sent down and these pulses of light are what brings your telephone to you and is in fact what is bringing my image to you this very minute."