Cite an experiment to show that sound needs a material medium for its propagation.
Sound is a mechanical wave which needs a material medium to travel.
It can travel through air, water, steel, etc. but cannot travel through vacuum. This can be demonstrated by the following simple experiment.
Experiment:
i) Suspend an electric bell inside a glass bell jar by passing the connecting wires through an airtight cork fitted at the mouth of the jar.
ii) Place the jar over a disc which has a pipe connected to a vacuum pump.
iii) When we turn on the switch, we hear the sound of the bell.
iv) Now, we pump out the air from the jar gradually with the help of the vacuum pump. The sound becomes fainter.
v) When most of the air has been removed, we hear a very feeble sound.
vi) When there is air inside the jar, sound travels through it to the wall of the jar. This makes the wall to vibrate which in turn, sends sound to us.
vii) When air is removed, sound from the bell cannot travel to the wall of the jar.
Hence, the sound needs a material medium for it's propagation.
