Science | Science Experiment of the Week 10 | 302 - Fiber Optic Water

Reflecting Light Beams - Why does light travel so well inside water, even when the stream is bent?

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This Week's Experiment - #302 Fiber Optic Water

These experiments are from Robert Krampf - The Happy Scientist

Today we put on Christmas music and decorated our Christmas trees. Yes, I said trees. We have two this year. One is a normal, green tree. The other is a silver tree with fiber optic lights. It changes colors and twinkles. As we were decorating, Lisa asked about the fiber optics, which led to this week's experiment. To explore fiber optics, you will need:

a metal food can
a hammer and nail
water
a flash light

Open the can and empty it. I used a can of peaches, which Lisa and I enjoyed as a snack. Rinse the can and use the nail to punch a hole in the bottom of the can. Check to be sure that the hole does not have sharp edges. Working over the sink, put your finger over the hole, and fill the can with water. Have someone turn off the lights for you. Turn on the flashlight and aim the beam down into the can of water. Remove your finger from the hole and look carefully at the stream of water. You should be able to see the light shining through the water. If not, move the flashlight a bit until you can.

Place your finger in the stream of water. What do you see? There is a bright dot of light on your finger where the water hits. This is light that is traveling down the stream. As the water level drops, you can tilt the can so that the stream goes out to the side, bending down. Even with the stream of water bending, you should be able to see the spot of light on your finger, as long as it is fairly close.

Why does the light travel so well inside the water, even when the stream is bent? Darken the room again. Hold your finger over the hole and fill the can with water. This time, hold the flashlight about 6 inches above the water and slightly to the side. Shine the beam on the water and look at the ceiling. There is a spot of light up there! It is reflecting from the water. When the light hits the surface of the water, some of it passes through, but some of it is reflected. If you had a waterproof flashlight and a very large container of water, you could hold the flashlight underwater, with the beam shining upwards towards the surface. Again, the surface would reflect some of the light, this time with the beam reflecting back into the water.

While we are talking about reflecting, imagine for a minute what would happen if you had a long tube-shaped mirror, with the mirror surface on the inside. If you directed the beam of the flashlight into the tube, the light would reflect from one side and then the other, bouncing around until it reached the other end of the tube. This would work, even if the tube was bent.

The same thing happens with the stream of water. When you shine the light into the can, some of it goes into the water flowing from the hole. When the light hits the side of the water stream, most of it reflects back, just as it would in our imaginary mirror tube.

Real fiber optics are long plastic fibers that look like clear fishing line. The plastic is very clear, letting the light pass through easily. The sides of the fiber act very much like the sides of the water stream, reflecting the light back and forth inside. Fiber optics are used for many things besides high tech Christmas trees. They are used to carry all sorts of information, from computer data to telephone conversations. Instead of transmitting these as electrical impulses through wires, they are sent as pulses of light, though the optical fibers.

If you don't want to go to all the trouble of punching holes in cans, you can do the experiment with just a faucet and a flashlight. Turn on the water to form a thin, constant stream. Hold your finger in the stream, about 4 inches below the faucet. Shine the beam of the flashlight onto the stream, from the side. You can even direct the beam upwards at an angle. You should still see a dot of light where the water hits your finger. It will not be as bright as with the can, but it should be bright enough for you to see in a darkened room.

Well, I have to wind this up now, as Mawra the cat is rearranging the decorations on the tree. Have a good week.

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