A cylindrical tube, open at both ends, has a fundamental frequency v. The tube is dipped vertically in water so that half of its length is inside the water. The new fundamental frequency is
(a) v/4
(b) v/2
(c) v
(d) 2v.
The correct answer is (c) v
EXPLANATION:
When a tube of length L open at both ends has a fundamental frequency ν, then v = V/2L where V is the velocity of the sound wave. When this tube is half dipped in water, the length of the air column becomes L/2 and it becomes a tube closed at one end. In such closed pipe the frequency of the fundamental mode ν' = V/4L'. Here L' = L/2, so ν' = V/(4*L/2) = V/2L = ν
Hence the option (c).