How fiber-optic media converter works?

There is a phenomenon in optics called "total internal reflection". Suppose a light ray is in some medium like glass and is about to exit into the air. If the ray is incident on the glass/air surface at a large enough angle then it gets totally reflected back into the glass. This phenomenon is used in fibre optics. Light is sent into the end of the fibre at an angle, so it travels a short distance and hits the side of the fibre glass tube. Because it is basically a glass/air surface it gets reflected back into the tube and ,of course, promptly hits the other side. But again it is reflected, rather then escaping into the air. So the light travels down the tube, bouncing off the sides as it goes. This is a really neat way of guiding the light along a path you want it to go. The "internal reflection" is highly efficient (meaning almost none escapes). The phenomenon only works when light is in a more dense medium (glass) then the surrounding medium (air). Another Words it doesent work if the light is in air and about to enter the glass. In that case some light gets reflected but most of it will travel into the glass. You might try this effect in a swimming pool. Some night take a flashlight under water. First shine the light directly up. It will have no trouble escaping. Now slowly turn the flashlight so the beam hits the surface at an angle. There will be an angle ,called the critical angle, at which the beam is all of a sudden reflected from the surface, back into the pool. The water surface has to be nice and still for this to work but you should be able to see the beam go thru this change. And a person outside should see the bright spot on the surface almost dissapear, since the light is no longer escaping from the water.