Experiment 5

Light at the End of the Tunnel

An Introduction to the Study of Fiber Optics


The objective of this experiment is to show that, because of internal reflection, light will travel down a glass tube.

Review of Scientific Principles:

Being a noncrystalline material, glass does not have grain boundaries to interfere with the passage of photons (light bundles). As long as the light waves hit the inside walls at less that the critical angle (the minimum angle that will allow light to be transmitted into the glass), most of the light will reflect off the side walls and continue through the tube.

The rods in a fiber optic system use a core inside a clad design. The core is made of a high purity glass with a larger refractive index than the outer layer. (A refractive index is a measure of the amount that light bends going into or out of a material.) The greater the difference in the refractive indices, the more light is reflected within the inner tube.

Most fiber optic systems use a laser as the light source due to its coherency and the fact that it can be controlled with high frequency pulses. The light pulses sent from one end of the fiber optic cable, are received and decoded at the other end to obtain the original information.


Much of the data sent over today’s communication networks is being carried by light pulses moving through fiber optics.

Time: 20 minutes

Materials and Supplies:

glass rod ( 5 mm +/- diameter; 15-20 cm long)

gas burner

small penlight flashlight

one hole rubber stopper to fit flashlight

General Safety Guidelines:

Be aware of the fire and hot glass.

Be extra careful when preforming the glass insertion.


1. Light burner and preheat the center of the glass rod until it gets soft.

2. Slowly bend the rod so that no bend has a radius of less than 2 cm. Make a continuous glass bend of your choosing. Avoid making sharp corners during bending.

3. Cool. Carefully insert one end of your rod into a one-holed rubber stopper. Put the stopper on the end of the flashlight.

4. Turn on the flashlight and observe the amount of light coming through the rod and the amount of light leaking out along the rod.

5. If time permits, try different radii in the curves. If you have access to a laser, try it in the tube as well.

Video Clip


1. What did you observe from shining the flashlight through the tubing?

2. What advantage does a laser have in this experiment over a flashlight?

3. Why was it necessary to form bends with at least a 2 cm radius?

Teacher's Guide to Experiment # 5 Light Lab


You might want to conduct the tests in a darkened room.

Answers to Questions:

1. Students should report most of the light exiting the other end of the tube. Only a small fraction of the light will come out the sides of the tube.

2. The laser light can be aimed along a straight line down the tube. The light from the flashlight will spread out and some will exceed the critical angle and escape out the side of the tube.

3. At smaller radii, the light will strike the walls of the tube at angles greater than the critical angle.

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