Question

In Fig. 7.15, A, B, C and D represent the test tubes each of height 20 cm which are filled with water up to heights of 12 cm, 14 cm, 16 cm and 18 cm respectively. If a vibrating tuning fork is placed over the mouth of test tube D, a loud sound is heard.

(a) Describe the observations with the tubes A, B and C when the vibrating tuning fork is placed over the mounth of these tubes.

(b) Give the reason for your observation in each tube.

(c) State the principle illustrated by the above experiment.

Answer 1

a) No loud sound is heard with the tubes A and C, but a loud sound is heard with the tube B.



(b) Resonance occurs with the air column in tube B whereas no resonance occurs in the air
column of tubes A and C. The frequency of vibrations of air column in tube B is same as the
frequency of vibrations of air column in tube D because the length of the air column in tube D is 20-18 = 2cm and that in tube B is 20-14 = 6 cm (3 times). On the other
hand, the frequency of vibrations of air column in tubes A and C is not equal to
the frequency vibrations of air column in tube B.


(c) When the frequency of vibrations of air column is equal to the frequency of the
vibrating tuning fork, resonance occurs.