A convincing demonstration of how fibre-optics work using a stream of water and a laser.

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• Total Internal Reflection
• Fiber optics

Theory:

Fiber optics are the leading method of data transport and is a direct application of total internal reflection (see the “Total Internal Reflection” demonstration). Individual fibers can be thought of as long, thin tubes – some can be smaller in diameter than a human hair! If these tubes are surrounded by air (with an index of refraction of n_a), the light traveling through it will never leave the tube as long as the incident angle of each reflection is greater than the critical angle, q_c= sin^-1 (n_a/n_f), where n_f is the index of refraction of the substance the tube is made of (see Figure 1).

As long as this criterion is met, water will work very well as a fiber optic, as shown in this demonstration. The index of refraction of water is 1.333 and air’s is 1.00029 (Hecht, p. 95). If a stream of water is used instead of a tube and it is surrounded by air, the critical angle is therefore,

q_c = sin-1 (1.00029/1.333)

q_c = 48.625°

The set-up of this demonstration ensures that the incident angle of the laser used is greater than this.

Apparatus:

• Optical bench
• mountable laser and its mount
• tube with one open end and smooth, circular hole punched out on one side
• duct tape to cover hole
• water
• container to catch the water leaving the tube
• lab jack

Figure 2: Water fibre-optic set-up

Procedure:

• Mount the laser and place it on the optical bench.
• Place the tube on the lab jack and align the tube and laser so that the laser is entering the tube directly opposite the hole and exiting through the hole. Remember not to look directly at the laser while trying to align it with the hole!
• Get the water-catching container in place.
• With the hole covered with duct tape (fold one side of the tape over before putting on the tube so that it is easily removable) fill the tube close to the top with water.
• Turn off the lights and remove the tape from over the hole. The stream of water will act as a fiber optic and the laser will follow it into the container.

SAFETY WARNINGS:

• NEVER look directly at the incoming laser. The best way to align it with the hole is to look through the clear plastic tube from behind and adjust its height accordingly
• Make sure the laser is securely mounted so that it doesn’t fall into any surrounding water!