Q1.
A space where there is no medium (matter) is referred to as vacuum. Would you hear sound in vacuum?
Answer
No. Sound cannot be heard in a vacuum, because a sound wave has no way to travel without particles.
The proof is the vacuum bell jar experiment (Fig. 10.7):
- An electric bell inside a sealed bell jar is switched on — the ringing is loud.
- A vacuum pump slowly removes the air. The sound gets fainter and fainter.
- Near vacuum: the hammer can still be seen striking the gong, but almost no sound is heard.
- Let the air back in and the sound returns, growing to its original loudness.
Why it happens: a compression is a region where particles have been crowded together. Fewer particles in the jar means fewer collisions to hand the crowding forward, so less and less energy is relayed to the jar walls and out to your ear. With no particles at all, there is nothing to compress and nothing to relay — so no sound wave can exist. Sound is a mechanical wave: it needs a material medium.
Tip: notice what stays visible while the sound dies — light reaches you from the bell throughout. Light is not a mechanical wave, so it crosses vacuum easily. That is the cleanest way to remember the difference.