Total Internal Reflection
or how optical fibers work

This experiment shows total internal reflection.
You can see the light from the laser pointer going through the bottle, and it goes right through the middle of this short piece of straw. The light bounces around inside the stream of water, it cannot get out until the stream breaks up, so the light ends up in the bucket.
Here is the version you can make yourself. In the box you can find:

→

  • Rubber stopper
  • Short piece of tube
  • 2 clothespins
  • Exacto knife
  • Laser pointer
You need to provide:
  • 1-liter soda bottle (*)
  • Cutting board
  • Books or magazines
  • Chewing gum!
(*) If you don't have a 1-liter soda bottle, other bottles may also work, but find one where
(a) the rubber stopper fits, and
(b) it is not too small, and
(c) it has a 'straight' section, so that you can shine the laser through, and
(d) it is sturdy (like a soda bottle).

First, remove the wrap-around label from the bottle.
Next, mark where you will cut a little hole. Choose the spot such that on the opposite side of the bottle, there is clear plastic (no leftover bits of label or glue.
In the example, the hole is about 2 inches up from the counter. Using the exacto knife, start cutting the hole, a little on the small side, and enlarge the hole carefully until you can push the little tube about halfway in. Chew some gum and use that to make a waterproof seal.
Now line up the laser pointer (or laser box). In the example, one clothespin is used to turn the laser on and keep it on, and half of the other clothspin makes a wedge so you can fine-tune the angle.

You need to find the right height, using a cutting board on the bottom (because water will spill on the counter and you want to keep the books or magazines dry), and then books or magazines so that the laser goes into the middle of the little tube
Ready to go!

Fill the bottle to the top with water, and put the stopper in. This will prevent the water from starting to run out (why?).

Put the bottle back to where the laser goes through the bottle and into the middle of the little tube.

Hold the bottle down and take off the stopper.

Watch the light end up in the bucket! If you look closely, you can see the laser beam inside the water stream, and bounce off the inside surface of the water, especially close to the bottle.



Looks even better after dark





(P.S. In all these years, this was the first time that chewing gum was the perfect material to use in a project.)
Hubert van Hecke
Last modified: Thu May 14 21:59:43 MDT 2020