CBSE Class 8 Answered

Activity to demonstrate that sound cannot travel through vacuum:

1. A ringing electric bell is placed inside an airtight glass jar containing air as shown in Figure (a). We can hear the sound of ringing bell clearly. Thus, when air is present in the glass jar, sound can travel through it and reach our ears.

2. The glass jar containing ringing bell is placed over the plate of a vacuum pump [Figure (b)]. Air is gradually removed from the glass jar by switching on the vacuum pump. As more and more air is removed from the glass jar, the sound of ringing bell becomes fainter and fainter. And when all the air is removed from the glass jar, no sound can be heard at all (though we can still see the clapper striking the bell) [Figure 6(b)]. Thus, when vacuum is created in the glass jar, then the sound of ringing bell placed inside it cannot be heard. This shows that sound cannot travel through vacuum (and reach our ears).

3. If air is now put back into glass jar, the sound of ringing bell can be heard again. This shows that air is necessary for the sound to travel from the ringing bell to our ears.