The sound waves need material medium to travel because they travel by the vibrations of particles of the medium. The material medium is gas, liquid and solid.
Sound waves are called mechanical waves as they need a material medium for their travel.
An activity described below tells that sound can not travel through vacuum. The meaning of vacuum is empty space or space where there is no air or there is no medium.
Activity –
We take an airtight glass jar connected with a vacuum pump. We place a ringing electric bell inside the jar as shown in figure. The sound of ringing ball is clear to us. Now, we remove air inside the jar by switching on the vacuum pump.
Observations –
Initially the sound of bell is clear. As more and more air is removed from the bell jar, the sound of ringing bell becomes weaker and weaker. When all the air is removed from the jar, there is vacuum in the jar and no sound can be heard.
Conclusion –
Sound can not travel through vacuum.
Questions:
(1)Give scientific reason why Sound can not be heard on the surface of moon.
(2)Fill in the blanks-
A student has put a ringing electric bell in a container and air is being removed from the container so that vacuum could be created in the container. After some time sound of the electric bell is not heard by the student. The speed of sound has become _______.
Answers:
(1) Sound can not be heard on moon because there is no material medium for travel of sound on moon.
(2) Zero (because there is vacuum created in the container)