Make a Light Fountain

Use this activity to illustrate light reflection, specifically total internal reflection, as well as how fiber optics work.

Materials

  • Clear plastic bottle with label removed
  • Duct tape
  • Blue painter's tape
  • Dark room
  • Sink or bucket
  • Thumbtack
  • Corkscrew
  • Flashlight

Directions

  1. Place a 2-inch piece of duct tape on the side of the bottle to create a "patch."
  2. Use the thumbtack to punch a hole in the center of the tape patch.
  3. Stick a piece of painter’s tape over the hole in the bottle. (Later, you will be able to pull off the blue tape without pulling off the duct tape.)
  4. Fill the bottle with water.
  5. Turn on the flashlight and turn out the lights.
  6. With one hand, hold the bottle over the bucket or the edge of the sink. With your other hand, hold the flashlight on the side of the bottle across from the hole.
  7. Remove the blue tape.
  8. How does the light enter the bottle and what does it do as it comes out of the hole in the side? Now place the tip of the corkscrew into the hole you already have made and turn it to make the hole a little larger. What do you see now? How about if you catch the water in another container like a bowl as it drains?

Explanation

The light ray inside the stream of water behaves as it would inside an optical fiber. Optical fiber works like this: you send a light beam into one end of the fiber and it comes out the other end, just as light travels through the stream of water in your experiment. It doesn’t matter if the fiber is straight, curved, or bent into loops; the light beam travels all the way through and comes out the other end. This is called total internal reflection. The light beam bounces around inside the fiber, reflecting back and forth off the walls. It doesn’t pass through the walls and out of the fiber, and it doesn’t stop until it comes out the far end.