a) Sound is defined as the form of energy which can be heard by us. Sound waves are longitudinal waves in air.
b) Consider a bell jar which is covered using a cork. When an electric bell is kept outside the bell jar the sound produced by the bell is heard. When the same electric bell is placed inside a bell jar, the sound produced by the electric bell is not audible to our ears. This explains that sound waves cannot travel in vacuum as there are no particles that can vibrate resulting in the production of the sound.
