Question 19: (i) An electric bell is suspended by thin wires in a glass vessel and set ringing. Describe and explain what happens if the air is gradually pumped out of the glass vessel.
​(ii) Why cannot a sound be heard on the moon? How do astronauts talk to one another on the surface of moon?

Answer: (i) Sound waves are longitudinal waves, which require a medium to travel. If the air inside the glass vessel is pumped out gradually then a vacuum is created around the ringing bell. As the sound waves generating from the ringing bell do not find any medium to travel in their surroundings, therefore the sound of the ringing bell fades and after some time it stops coming.

(ii) Sound waves are longitudinal waves, which require a medium to travel. On the moon, there is no atmosphere, so in the absence of atmosphere, the sound waves find no medium to travel from one place to another. That's why no sound can be heard on the surface of the moon. The Astronauts use the radio waves to communicate on the surface of the moon.