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Water Fibre Optics

Public summary: 

Find out what happens when light is trapped inside a stream of water. Make a beam of light bend round corners and see how fibre optics are helping to change the world!

Constraining light in a stream of water, making it work like an optical fibre.
Useful information
Kit List: 
  • Large bucket
  • Lemonade bottle with a hole drilled in it
  • Torch (the more powerful the better)
  • Optical Fibre Christmas Tree
  • 1m length of 3mm Optical Fibre
  • Glass Bowl (Steal from Oli+Pyrex)
  • Spoon
Packing Away: 

Dispose sensibly of the water! Pack away into slug bubbles box.

Frequency of use: 

Borrowed from Dave's Naked Scientists explanation:

In a nutshell

A stream of water is used to bend light and demonstrate how fibre optics work.

What to do

  1. With the bottle empty, ask where they expect the light to go when the torch is switched on. Most will say in a straight line. Show them they are correct - it's useful to have a wall to point the torch at.
  2. Put your finger over the hole and fill the bottle up with water.
  3. Shine the torch through the bottle at the back of the hole.
  4. Remove your finger from the hole and move it down the stream of water, showing how the light no longer travels in a straight line.

If you can, put a short straw into the hole - you'll get a smoother flow of water than just through the hole alone.

You should notice a spot of light on your hand while it is in the stream of water even though it must have gone around a corner to get there. It tends to work best when the water comes out quite slowly.

What you need to know during the experiment

To understand what is going on here it helps to do another experiment. Fill a transparent bowl with water, put something in the bowl and then look upwards at the bottom of water.

If you look at the bowl from the top you can see the spoon at the bottom.

Looking upwards in the bowl of water you see a reflection of the spoon from the surface of the water. The water is behaving like a mirror.

So light will reflect really well off the inside surface of water at a relatively small angle.

This means that if you shine the light into a tube of water whenever it meets the side it is reflected so the light stays within the water until it hits your hand lighting it up. This happens even if the water goes around a corner.

We say there is total internal reflection if no light is refracted.

Want to know more?

Why do you get such a good reflection from the surface of the water?

Light goes more slowly in water than in air and whenever light changes materials and the speed changes it will be bent (refracted). When it moves from a slow material (like water) to a faster one (like air) it is bent towards the surface.

Optical Fibres

If instead of making the tube out of water you use very very pure glass and pull it to a thin flexible fibre, when you shine light in at one end it will come out of the other. By getting the right design of fibre the light can travel through up to 50 km of fibre and still be detectable. You can then send signals through the fibre by flashing the light on and off again a bit like morse code, and if you can flash the light very fast you can transfer huge amounts of information. Speeds of data transfer can now reach 1.125 Terrabits per second (2016) down a long-distance optical fibre connection ( Because they are so good at transmitting data optic fibres move most of the data around the world (internet traffic, phone calls etc.) and you are almost certainly reading this via one.

If you make the tube out of plastic rather than glass it is more flexible and safer, and you can use it to make the artificial Christmas trees with the tiny pin pricks of light.

In the box there's also a length of optical fibre which can be used to show exactly how they work, but be careful with it as it is fragile. Optical fibres have become very common and cheap and many products use them, some artificial Christmas trees use them so the end glows. Some have side glow as well, this happens when there isn't total internal reflection and some light is still refracted.

Risk Assessment
Date risk assesment last checked: 
Thu, 30/01/2020
Risk assesment checked by: 
Conor Cafolla
Date risk assesment double checked: 
Thu, 30/01/2020
Risk assesment double-checked by: 
Beatrix Huissoon
Risk Assessment: 

Pouring water out of a lemonade bottle with a hole in it shining a torch into it and looking at where the light ends up:

Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Water There is quite a lot of water involved, so if a surface is vulnerable to getting slippery when wet it could be a slip hazard. All 4 3 12 If the floor is vulnerable to slipperiness, clear up any spills. Use wet floor sign if necessary.
It may be worth laying down newspaper in the area around the bucket.
Try to do the experiment so as to minimize the amount of water getting on the floor in the first instance.
Call a first aider in the case of an injury.
1 3 3
Bucket of water If using the large bucket of water, note that it is very heavy to carry - could cause back issues and/or is susceptible to drop and spill water everywhere, leading to the risks described above. Demonstrator 3 3 9 Take care when carrying the large bucket of water, and seek assistance if necessary.
Use standard lifting techniques (e.g. lift with your knees, not your back) when carrying heavy objects.
Clear any spills either as a result of outright dropping the bucket, or from water sloshing out during transfer of bucket.
In the event of an accident call a first aider.
1 2 2
Torch Electric shock risk with water. All 2 3 6 Electrical torch to be well insulated from water - protect bodywork and bulb area.
Take care not to drop the torch in the water.
Turn off torch if in contact with the water.
Call a first aider in the event of an accident.
1 3 3
Optical fibre Optical fibre could smash, creating glass shards and cutting people. All 3 3 9 Careful handling of the fibre and not to allow children to play with it. Clear up any broken fibre quickly and dispose of it appropriately. Close the experiment if the fibre smashes.
Call a first aider in the case of an injury.
1 3 3
This experiment is sometimes run in a darkroom, see separate risk assessment.
Publicity photo: