I have collected my data, and found that the frequency indeed became lower as more water was added to the bottle. This supports my hypothesis. What I'm having trouble understanding, however, is the reason this happens.

From my research I found this answer on the Physics Stackexchange Forum, which essentially states the following:

The bottle acts as an air column for the vibrations in the air. Changing the volume of water changes the length of this air column. Therefore it will contain a gradually changing magnitude of the wave's wavelength. This, through the formula of f=v/λ, affects the frequency.

I don't really understand though whether this is actually saying the frequency becomes higher or lower when the volume of water is increased. Furthermore, in my case, is it actually the air that is vibrating, or the glass? What would this mean for my hypothesis?

I know that the hypothesis is correct, because my data clearly shows a downward relationship between volume of water and resonant frequency. I simply can not find what the reasoning behind this is. I know that this is probably a lot to ask, but I would

*really*appreciate some help on this, as I am determined to do well in this assessment and it contributes greatly to my final grade. Thank you!