Make a Light Pipe

By James E. Hyer

Every day, many messages are sent using pulses of light.

During most long-distance telephone calls, the sounds of spoken words are changed into signals made up of light pulses. These signals travel from one place to another through miles of “light pipes,” thin clear fibers that can curve around corners. At the end of the trip, the message is changed back into the vibrations that we call sound, and the listener seems to hear the caller’s voice.

You can make a simple light pipe and use it to transmit light along a curve. You will need a flashlight and a tall glass jar with a metal screw cap. The jar should be just a little bigger around than the flashlight.

You might need help from an adult for the next step. Punch a large nail hole in the center of the cap and a smaller hole near the edge. (The smaller hole will let air into the jar as you pour a stream of water out through the larger hole.)

Make a long tube by wrapping layers of dark paper around the jar. Brown grocery-bag paper or black construction paper works well.

The tube must extend far enough below the jar so that later you can put the head of the flashlight inside the tube beneath the jar. Fasten the paper with tape. Fill the jar almost full of water, and screw on the cap.

You will see the effect of this activity best at night or over a sink or tub in a dark room.

Turn on the flashlight and put it inside the tube, pointing it upward against the bottom of the jar. If light leaks out around the cap, cover the space with more paper or dark tape. Now the only light you can see comes out in a straight path through the nail holes in the cap.

With the flashlight still on inside it, turn the tube almost on its side so that a small stream of water flows out through the large hole in the cap. The stream of water seems to glow. Much of the light follows that curving stream of water. You have made a light pipe.

How It Works

How can rays of light bend? The answer is that they really do not. They travel in many straight lines, bouncing around inside the water stream. When a ray of light is traveling through water and hits the surface at a small angle, the surface acts like a mirror. Your bending stream of water works almost as if it were a bent glass tube with a mirror all over the inside. The light moves along the pipe, reflecting off this mirrorlike interior again and again.

The light pipes used to transmit telephone calls and computer information are made of materials that are much better than water for piping light. But all of them use the same idea.