In this demonstration, we will use a few handy materials to show how a stream of water behaves like a fiber optic cable.
- An empty 2L bottle (preferably clear with a cap)
- Electrical tape
- A small nail
- Rhodamine b dye
- Blue or Green Laser Pointer
- Get an empty 2L bottle and use the nail to a make small hole in it about 4 inches from the bottom. The hole in this demonstration was made by heating a nail over a candle flame, and easily poking it through the plastic container.
- Put a small square of electrical tape over the hole and fill it up with water. If you still have the cap, put the cap on. The electrical tape is not perfect for keep the water in, but with the cap, water will stay in the bottle due to pressure.
- Add several drops of Rhodamine b dye. This will dye the water pink.
- Use a blue or green laser pen and aim it through the solution. Either color should show an orange line going through the solution. Be very careful to be sure that no one is the path of the laser beam, before or after the bottle. Laser pens given off intense light and can cause serious damage if it gets in your eyes.
- Remove the piece of tape and take off the cap. Water will flow in a bending stream. Aim the laser through the hole from the opposite side of the bottle.
As water comes out of the hole it is a tight stream, because it has very high surface tension. It bends because gravity is acting on it. Eventually the water breaks up into a spray as gravity overcomes the surface tension.
While water is in a nice tight stream, it acts like a fiber optic cable. Laser light in the stream bounces back and forth and bends with the water. As the water breaks up, it no longer acts like a good fiber optic cable, and you might notice some blue or green light at the end.
The reason the pink solution appears orange is due to fluorescence. This is when a molecule absorbs one kind of light (like blue) and emits a lower energy light (like orange). Most of our highlighters use fluorescent inks, and appear very bright when they are hit with white light. Without a fluorescent molecule like Rhodamine, it would be difficult to see the laser light in the water. That is because water does not scatter or absorb visible light (this is also why it appears clear).