Question

A tuning fork of frequency 340 Hz resonates in the fundamental mode with an air column of length 125 cm in a cylindrical tube closed at one end. When water is slowly poured in it, the minimum height of water required for observing resonance once again is _cm. 

(Velocity of sound in air is 340 ms^-1)

Answer 1

Correct answer is 50

Assumption: Ignore word "fundamental mode" in question.

 First resonating length = λ/4 = 25 cm

Second resonating length = 3λ/4 = 75cm

Third resonating length = 5λ/4 = 125 cm

Height of water required = 125 - 75 = 50 cm