Communicating with light – fibre optics

Activity 1

Optical fibres as 'light pipes'

Light travels in a straight line. But light sent through optical fibres can go around corners. You can model the effect of optical fibres in this experiment.

Materials (for the class)

  • solid glass rod about 50 to 60 centimetres long, with a bend in the middle
  • 2 clamps and stands
  • sheet of cardboard (approximately 50 × 50 centimetres)
  • torch
  • piece of white paper (A4 or smaller)

Procedure

  1. Put the glass rod into the clamp on the stand.
  2. Make a hole in the centre of the sheet of cardboard and slide it over the end of the rod. The cardboard will act as a shield against light that isn't focused down the rod.
  3. Focus the torch on one end of the glass rod, and clamp it into position. (The glass of the torch can be touching the glass rod.)
  4. Hold a piece of white paper a short distance from the other end of the rod.
  5. Observe the beam of light on the paper.

Questions

  1. Given that light always travels in a straight line, where would you expect the light from the torch to land?
  2. Explain why the light follows the bend in the glass rod.
  3. There is only a weak beam of light transmitted through the glass rod. Why?

Teachers notes

Preparation

Obtain a rod of glass of about 3 to 5 millimetres in diameter. Put a 30-40o bend in it by heating the middle of the rod with a Bunsen burner, until the glass softens.

You can use a small torch with a diameter similar to the diameter of the glass rod.

If you use a larger torch, you may need to use black tape or paper around the space between the light and the rod to reduce the amount of light that is not focused on the glass rod.

A small halogen lamp could be used instead of a torch, but this will get too hot to allow you to wrap the intervening space with tape or paper.

The results are easiest to see if the room is very dark, so use a darkroom if one is available.

Try the following variations to focus more light down the rod:

  • Remove the glass of the torch and put the rod as close as possible to the bulb.
  • Use a lens to focus the light onto the end of the rod.

This experiment could be done in small groups, if you have enough equipment.

Answers to questions

  1. Normally the beam from the torch would land on a spot directly in front of the torch.
  2. When light travelling through the glass rod meets the air-glass interface at a small enough grazing angle, the light is reflected back into the rod and none escapes.
  3. Much of the light from the torch has been absorbed by the glass rod. All glass absorbs light. For example, when you look through a window pane, only about one half of the external light is visible through the pane. If the pane of glass was half a metre thick, much more light would be absorbed.

With new glass that has been developed for optical fibres, light can travel more than 10 kilometres before 90 per cent of it is absorbed. This is a big improvement over ordinary glass. When light travels through ordinary glass 90 per cent of the light has been absorbed after only about 20 metres.

Students may notice that the light travelling down the rod is coloured. Glass does not absorb all the wavelengths of light equally (eg, Pyrex glass absorbs blue and red wavelengths, transmitting yellow-coloured light; ordinary soda glass transmits green light best). Infrared light is used to send messages down optical fibres because glass absorbs least in the infrared part of the spectrum.

Even with ideal conditions, a fibre optics network requires an amplifier every 10 or 20 kilometres of optical fibre to boost the light signal, but this distance is being increased as glass technology improves.

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Posted May 1997.