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Flowing Fibre Optics

Normally fibre optics are hidden underground, but this experiment uses water to see the light

Australian Curriculum links

  • Science > Physical Sciences > Year 5 > ACSSU080

You’ll need

  • 1.25L soft drink bottle with label removed
  • Drill (optional)
  • Water
  • Bucket
  • Bright torch
  • Masking tape

Try this

  1. Make a small hole in the side of the bottle near the base. Using a drill is recommended. Place some masking tape over the hole.
  2. Fill the bottle with water.
  3. Place the bottle on a table, setting up the bucket on the floor so that the water stream from the hole will go into the bucket.
  4. Remove the tape so that the water begins to flow from the bottle.
  5. Hold the torch so that it shines through the bottle from the side opposite the hole. While keeping the torch in place, hold your hand in the stream of water and move it up and down. You should see a spot of light on your hand. If you don’t, try moving the torch slightly.

Further investigation

  • Before starting this experiment, look at the way the light from the torch is emitted. Does it bend or go in a straight line?
  • Try shining the torch in a mirror. What happens to the light?
  • If you have a laser pointer or low-energy laser, try shining it into the water stream instead of the torch. Can you trace the laser through the water stream?

What’s happening?

When the stream of water leaves the hole in the side of the bottle, light entering the water from the torch also passes into the jet of water. This light is trapped inside the jet of water (it’s doesn’t dissipate into the air around it) because the internal surface of the water jet acts like a mirror, and reflects the light back into the water jet whenever it reaches the jet’s surface. This is called the ‘principal of total internal reflection’.

Real world links

Fibre optics cables use this principal to transmit light over long distances. They are made of very thick threads of glass, whose internal surfaces act like mirrors, just like the jet of water. These mirrors contain the light, and allow it to pass around curves and corners wherever the glass bends.

Computers use lasers to transmit very short pulses of light (billions of pulses per second per second) via fibre optic cables to transfer information from one place to another. Some new fibre optics systems can channel many different coloured beams of light through a single cable, to transfer information even more efficiently.