ICSE Class 7 Answered

Connect the bell to the battery so that it starts ringing. Arrange the bell jar around the bell and connect it to a vacuum pump.


Start the vacuum pump so that it starts evacuating air from the bell jar. Keep on observing the sound of the ringing bell all the time.
We observe that we keep on hearing the sound of the bell after the bell jar is kept over it. As the air inside the bell jar is slowly evacuated, the sound becomes dimmer and dimmer after sometime we cannot hear it at all.
But the bell still vibrates.
Thus we conclude that the ringing of  bell reaches us through the air in the bell jar.
When the vacuum pump removes air from the bell jar, the sound does not reach us even thought the bell is vibrating.
Thus, we can say that sound cannot travel through vacuum and requires medium for propagation.