Tyndall Effect Explained
(ripple tank demo)
NOTE: If you are not a Physics teacher you will have to contact a Physics teacher in your building for help borrowing the equipment and setup. They might be a "stuffy group", but they are usually helpful when pushed.
You will need the following equipment:
1) Ripple Tank (try to see if you can get one that fits on an overhead, but if you can't any will do);
2) Variable Speed Wave Generator (you don't need anything really fancy, as you will be making "straight waves" and not "point source waves");
3) Power Supply for the wave generator;
4) Ripple Tank Light Source (not required if you were able to get an "overhead tank");
5) 1/2" (wood) dowel rod, 1/4" dowel rod, 1/8" dowel rod, and some toothpicks.
Cut 4 pieces of 1/2" dowel rod so they are long enough to fit across the ripple tank. The remaining 1/2" dowel, 1/4" dowel, 1/8" dowel, and toothpicks you will use to make "comb teeth". Make 4 combs (one for each "tooth" size) by gluing about 5 teeth each spaced about ~2.5 cm (or ~1" apart) to the 1/2" longer dowel rods. You will want to place the teeth in the center of the long 1/2" dowel. The length of the teeth is not important, but they should be a little longer than the depth of the tank. These combs will become your "observation objects" (like a microscope slide).
If you did not get an "overhead" ripple tank, mount the light source below the tank for projection onto the ceiling rather than above it (which is normally done). Practice making straight waves of different wave lengths and adjusting the light source for best class viewing. Remember the faster you set the wave generator the shorter the wave length of the waves and the slower you set the wave generator the longer the wave length of the waves.
Place the combs in the tank one at a time starting with the largest. You should notice a definite "shadow" in the wave from the teeth of the comb. The shadows from the teeth should be smaller and smaller as you use smaller and smaller teeth. The "toothpick comb" should give you no noticeable shadow from the comb teeth. This is because the toothpicks are much smaller than one wave length so the water waves can't "see" the toothpicks. This is basically the same as the Tyndall Effect for light. The particle size of colloids are on the order of the wave length of light so you can "see the gelatin" in water. A molecule of sugar is much smaller than the wavelength of light so you can't "see the sugar" in solution.
Perhaps the most interesting effect may be obtained with the 1/8" teeth. It should be possible to adjust the wavelength so that the teeth ARE observable for short wavelengths, but NOT observable for long wave lengths.