Through the Looking Glass?


Instead of using electricity through wires, fibre optic cables use light traveling through a clear fibre to carry phone signals, etc. Even though the fiber is clear, the light stays inside until it reaches the end. Then it emerges from the fibre, even if it has twisted and turned along the way. How can it do that? To find out, you will need:

  • a small mirror
  • a container of clear water at least 5 or 6 inches deep

Hold the mirror so that it is about 3 inches under the surface of the water, with the shiny side facing upwards. Hold it flat, so that you can look down into the mirror, through the water. You should see your reflection in the mirror, staring up at you. Stick your finger into the water, so that you can see its reflection in the mirror. You should see the reflection of your entire finger. You can see the part that is under the water and the part that is above the surface.

Slowly tilt the mirror. As the angle increases, suddenly, the part of your finger that is above the water will disappear. You can still see the part that is under the surface, but it looks as if it is sticking through a mirror. If you lift your finger, it will seem to vanish into the mirrored surface. If you are a fan of the science fiction show Stargate, this will look very familiar. I wonder if the person that designed the animation for their special effects ever played with this experiment.

Why does the surface of the water suddenly change from transparent to a mirror? It has to do with the way that light bends as it moves from one substance to another. Fill a clear glass with water. Looking from the side, stick your finger into the water. It seems to be broken, with the part below the water moved to the side. It looks that way because as light passes from the water to the glass, and from the glass to the air, it is bent. That bending of the light moves the image of your finger to the side.

If the light is coming straight through, the bending does not have a big impact, but if the light is coming from an angle, then the bending is more important. When the angle is small enough, then the light is bent enough so that it is directed away from the surface, reflecting back into the water.
The point where this happens is called the critical angle. When you turn the mirror to the proper angle, the light that is being reflected from it is at a sharp enough angle that it does not pass through the surface. When your finger is in the water, you can see it, but when it is above the water you cannot.

This is the basic idea of fibre optics. As the light goes through the clear fibre, its critical angle keeps reflecting the light back from the sides. Almost 100% of the beam of light remains inside the fibre, even if it is bent. At the end of the fibre the light is moving directly towards the surface, making an angle greater than the critical angle, so it passes through easily.

From Robert Krampf's "Experiment of the week" selection - highly recommended!

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