A chain of events occur when you move your
finger across the rim of a glass and make sound.
First thing you need to know is that material
has a frequency at which it vibrates. This is
called its resonant frequency. Your example
of singing glasses is a good way to explain
If you run your wet finger around the rim of
the glass too fast, the glass will not make any
sound. And if you run your wet finger around the
rim of the glass too slowly, the glass will not
make a sound. In order for the glass to make
noise you will have to move your wet finger around
the rim of the glass at the same frequency (or
speed) as the resonant frequency of the glass
(or material). Carefully, try this at home with
glasses to see how fast you need to move your
finger around the glass in order to make noise.
When you run your finger around the glass,
this gives energy to the molecules in the glass
making them vibrate. When individual
molecules in the glass vibrate it makes the whole
glass vibrate. This vibration then transfers to
the sir molecules and that is what makes up the
sound waves that you hear.
When you change the amount of water in the
glass you are adjusting the mass of the system
(the water and the glass). The more mass you
have in the glass the less the glass molecules
vibrate, the less the molecules vibrate the less
energy that is in the system, the less energy
causes the air molecules to have a lower frequency
in which you can tell from the pitch of the sound.