Resonance in a closed tube with adjusted length

Steps:

• Initial setup: Tube acts as closed pipe at water surface; first resonance at fundamental frequency where length L = λ/4.
• Frequency fixed after first resonance; louder sound indicates standing wave with node at water (closed end) and antinode at open end.
• Water removal increases effective length L' to satisfy next resonance at same frequency: L' = 3λ/4.
• New standing wave spans three quarter-wavelengths, with node at new water position and antinode at tube top.

Why C is correct:

• Depicts 3λ/4 pattern with node at closed end (water) and two additional nodes/antinodes fitting closed-pipe overtone.

Why the others are wrong:

• A: Shows λ/4 fundamental, unchanged from initial setup.
• B: Indicates open-pipe pattern (λ/2), mismatched for closed end.
• D: Displays higher harmonic (5λ/4) or incorrect node placement.

Final answer: C