Question

 Describe a simple experiment to show that sound cannot propagate through vacuum.

Answer

Take a small bell jar having a rubber cork fitted in its neck. Drill a small hole centrally through this rubber core. Suspend a small bell inside the jar with the help of a glass rod. Let the upper end of the rod project out from the hole in the rubber cork and fit this cork firmly in the neck of the bell jar. Close the small opening around the glass rod with an adhesive solution so that the rubber cork closes the jar’s neck in an ‘air tight’ way. Shake the jar so that the bells start vibrating. You will be able to hear the sound of the bell quite clearly.

Now, connect the bell jar to a vacuum pump and start evacuating the air from it. You will no longer hear the sound of the bell when the jar is shaken. You may see the bell vibrating but you do not hear its ringing sound. This is because of the absence of air in the jar. This shows that sound needs a medium for its propagation.