  We'll describe reflection and refraction of light, and show you a simple experiment that will illustrate how fibre optics work. We'll also explain more about fibre optics and how it is used. But first, let's look at total internal reflection. When light travelling in a transparent material meets the surface of another transparent material, two things happen; some of the light is reflected and some of the light is transmitted into the second transparent material. If the light meets the surface of the first material at a large enough angle, (called the critical angle), almost all of it will be reflected. That's why when you look down onto the surface of water, sometimes you can't see the fish below. The light rays from the fish hit the surface at such a big angle that they are all reflected back into the water, and no light from the fish reaches your eyes. This is called total internal reflection.  You may have experienced something similar from the fish's point of view, if you've ever opened your eyes underwater and looked at an angle towards the surface. Light from above the surface can't get through ... you can't see out. What you see instead is a mirror-like surface that represents the light from below reflecting back down into the water. Of course, the surface of water is rarely flat. Waves and ripples, as well as temperature differences, will make the critical angle for total internal reflection change from place to place, and from second to second. When you're searching for a glimpse of the fish below, some of the light from the fish may indeed get transmitted towards your eye as it hits the water at an angle less than the critical angle. But what usually happens is that this faint glimpse of the fish gets overwhelmed by the glare off the water, which is much brighter. Polarized sunglasses will filter out this glare and allow you to see whatever light from the fish manages to make it through the water's surface. However, if the fish is at an angle greater than the critical angle, you won't see any light from it at all. [Actually, total internal reflection is really just refraction. Physics 20 students might like to find out more here.] Now let's look at how total internal reflection can be put to use. First we'll show you a simple experiment you can do to illustrate sending light through a pipe.   In our experiment, the pipe will be a stream of water.
You'll need a flashlight, some thick tape, a baby food jar, and some water. You'll also need a hammer and nail to poke two holes in the jar lid.You can eat the baby food if you want. Personally, we weren't that hungry. Just make sure the jar is empty and clean. 
Fill the jar with water and put the lid on tightly. Tape the jar to the flashlight with the lid outwards. (The tape is to keep light from escaping from the jar. We used white tape, and had to wrap it many times; duct tape or black electrical tape would be a better choice).Now use the hammer and nail to poke two holes in the lid, one at the top and one at the bottom. Air will be able to get in the top hole, allowing water to escape through the bottom one. Turn all the lights out, turn on the flashlight, and start to pour the water out of the jar. 
You will see some light escaping from the hole. You will also see some light escaping from the stream of water ... it will be visibly lit up. This is light that wasn't reflected internally.However, what you will also see if you look closely is that the puddle of water that forms at the bottom is also lit up! This happens because some of the light travelling inside the tube of water experienced total internal reflection every time it hit the edge of the water, and wasn't able to escape. It just got bounced back and forth inside the tube of water all the way down to the bottom. What we've just done is sent light down a 'pipe'. Water isn't a very efficient light pipe. A lot of light escaped into the air. If we could somehow send light through a more effective pipe, we could send signals with it. That's what fibre optics does, with glass tubes instead of water. Let's find out more ... move on to page two >>> |