Elenco Fiber Optics Voice Data Kit - Manual (Page 8)

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The light-carrying portion of an optical fiber must be protected
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In this activity you will observe how changing the optical density (refractive index) of the
material surrounding the acrylic rod will affect the rod's light-transmitting ability.
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Penlight with batteries
Black rubber penlight boot
3 mm (1/8 inch) acrylic rod bent into "U" shape from Experiment 2
Pan of water about 20 cm (8 inches) wide and 5 cm (2 inches) deep*
* Not contained in this kit.
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• Turn the penlight on. Insert one end of the U-shaped rod into the black rubber boot (if you
removed it previously.) Slowly immerse the bent
portion of the rod in the water, but be careful to keep
the penlight and rubber boot out of the water.
• As you immerse the rod in the water, observe the
amount of light coming out the end of the rod.
• Now dim the room lights, immerse the rod in water
and again observe the light as it travels through the
rod. Can you see light escaping from the rod? Where
does the light go? Turn the penlight off.
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As the bottom of the U-shaped rod is immersed, the
amount of light coming out the far end of the rod decreases.
When the room lights are dimmed you should be able to see
light escaping from the plastic rod by looking at the bottom of
the pan.
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The decrease in light from the rod end is caused by the
change in optical density outside the rod when it is dipped in
water. The optical density of water is closer to that of the rod
than the optical density of air; therefore, it doesn't trap light
as well. When the light in the rod encounters the water,
some of it escapes and travels to the bottom of the pan. The
U-shape in the plastic rod increases the amount of light escaping when it is immersed in water.
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"Optical density" as we have described it previously may seem a little vague. The scientific term
for optical density as we have applied it really is "refractive index" or index of refraction. The refractive
indices of the three materials that you worked with in this experiment are shown in the table at the
right.
You might now ask: What good are optical fibers if their ability
to transmit light can be affected by conditions around them? If this
were actually the case, they would not be very useful. Most fiber
optics used for commercial applications are manufactured with a
coating around the central light-carrying portion so that external
conditions do not affect them. This coating is called "cladding" while
the central "light-carrying" portion is called the "core". A fiber's
cladding always has a lower refractive index than the core.
1154
Water 1.33
Acrylic
(plastic) 1.45
Air 1.0