A tuning fork of frequency 340 Hz is sounded above a cylindrical tube 1 m high. Water is slowly poured into the tube. If the speed of sound is 340 ms^{- 1}, at what levels of water in the tube will the sound of the fork be appreciably intensified? 

Frequency of sound emitted by a closed pipe of length L in the fundamental mode is v = $\frac{\mathcal{v}}{4\mathrm{L}}$ . 

For resonance v = v' where v' is the frequency of the tuning fork. Thus 340 = $\frac{340}{4\mathrm{L}}$ or 4L = 1 m or L = 25 cm. 
The next resonance occurs at $\frac{4}{3}$ L = 1 m or L = 75 cm.