Physics

When a tube is partially submerged, the water acts as a closed boundary—the air column can no longer vibrate below the waterline. The key insight is that the effective length of the vibrating air column has been cut in half, and since fundamental frequency is inversely proportional to length ($f \propto 1/L$), the new frequency will double. Think of it as the same tube suddenly becoming much shorter, forcing the standing wave to compress and oscillate faster.

When a tube is partially submerged, the water acts as a closed boundary—the air column can no longer vibrate below the waterline. The key insight is that the effective length of the vibrating air column has been cut in half, and since fundamental frequency is inversely proportional to length ($f \propto 1/L$), the new frequency will double. Think of it as the same tube suddenly becoming much shorter, forcing the standing wave to compress and oscillate faster.