Elenco Fiber Optics Voice Data Kit - Manual (Page 11)

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The first demonstration of the basic fiber optic principle
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In 1870, before members of the prestigious British Royal Society, John Tyndall showed how a
light beam could be guided in an arcing stream of water. Tyndall shined a bright light into a horizontal
pipe leading out of a tank of water. Then, when the water was allowed to flow out and downward in
an arc, light rays traveled inside the water until they were broken up by the water striking a collection
pan. With the help of fiber optics you will duplicate this historical experiment.
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Penlight with batteries
Black rubber penlight boot
2 mm (.08 inch) diameter optical fiber, 50 cm (20 inches) long
Empty 1-gallon plastic milk container*
Scissors*
Ball point pen*
Water*
* Not contained in this kit.
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• Using a scissors, cut a hole in the milk container
opposite the handle, large enough to insert your
hand.
• With a ball point pen puncture a hole about 3 mm
(1/8 inch) in diameter in the milk container below
the handle, opposite the opening you cut,
approximately 2.5 cm (1 inch) from the bottom.
• Turn the penlight on. (Insert one end of the
unjacketed 2 mm fiber into the rubber penlight
boot if you had removed it.)
• Take the milk carton, penlight and optical fiber to
your kitchen or bathroom sink. Dim the lighting in the room as much as possible while still
having enough light to see.
• Fill the milk container with water while holding your finger over the hole made by the ball point
pen, so no water can escape.
• Position the milk container at the edge of the sink so the puncture hole is over the basin while
continuing to cover the puncture hole with your finger.
• With your other hand insert the 2 mm fiber through the opening that you cut in the milk
container, and into the puncture hole. Remove your other finger.
• Observe the light beam after it leaves the fiber end and stream of water. Do you see the light in
the water stream?
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The light will leave the 2 mm optical fiber and follow, or be guided, by the stream of water to the
bottom of the sink.
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The light is guided by water, just as light was guided in the previous experiment, because air has
a lower refractive index than water, which traps light inside of it.
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Tyndall knew light was trapped temporarily inside the stream of water, but he could not explain
why. Today, using a combination of mathematics and science, the explanation is very straight forward.
Tyndall's work is significant because it marks the first recorded confirmation of the scientific principle
which forms the basis of all fiber optic products today.
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