Why Does Filling a Glass Affect Its Sound Wave Pitch?

[daniel_i_l]

If you fill a glass with water and hit it on the side with a knife then you get a higher pitch the more you fill it up. Why is this? When you hit the glass you aren't really making the air in the glass vibrate as you do when you blow over the top of a bottle, you're just making the glass vibrate. Maybe the amount of water changes the resonant frequency of the glass itself? Any explanation would be helpful.
Thanks.

 

[malawi_glenn]

the glass sets the air in motion, which propagates to our ears.

 

[russ_watters]

Maybe the amount of water changes the resonant frequency of the glass itself?

That's the answer. The water damps out vibrations , so it is only really the glass above the water that is vibrating. And just like a drum or a glockenspeil, smaller means higher in pitch.