This experiment is the result of things that I noticed while shooting the video for “The Singing Glass.” It is always fun to find a new experiment while you are doing an old one.

To try this, you will need:

3 thin glasses (be sure they are made of glass)
some water

Fill one glass about half-full of water. Then wash your hands very well. Be sure to use soap, as you want to remove all the oils from your skin. Then dip your finger into the water, and begin rubbing it lightly around the rim of the glass. When you get the right amount of pressure, you should hear the glass begin to make a ringing sound. With practice, you can produce a very nice tone. To understand why the glass sings, watch the video, or read the text version that is on the same page. For this experiment, we are more interested in the water.

While the glass is "singing", watch the surface of the water. You will notice lots of tiny ripples in the water. As the glass vibrates, it causes the water to vibrate too. That produces waves on the surface of the water. Notice the tone and the size of the waves.

Fill another, similar glass about one fourth of the way with water. Fill the last glass three fourths with water. Again, rub your finger around the edge of the glasses. You will notice that the glass with less water has a higher tone. The one with more water has a lower tone. Then carefully watch the waves. The glass with the higher tone has small waves that are very close together. The half-full glass has a medium tone, and waves that are larger and farther apart. The glass that is three fourths full of water produces the lowest tone, and has waves that are very large, and much farther apart. Why?

Different tones are produced by sounds with different wavelengths. The shorter the waves, the higher the tone. So very high tones have a very short wavelength, while low tones have a long wavelength. That is exactly what we are seeing with the ripples in the water.