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💡 Silence of space
Pump the air out and the ringing bell falls silent — even though you can still see it ringing.
What does the bell jar experiment show?
- An electric bell rings inside a glass bell jar .
- As air is pumped out, the sound gets fainter ; near vacuum, almost no sound is heard.
- Let the air back in and the sound returns — so sound cannot travel through vacuum .
In outer space there is a near vacuum, so sound cannot propagate. That is why astronauts on spacewalks cannot directly hear each other or the clanking of metal — they talk through radios fitted into their spacesuits.
NCERT Question 8 — In a movie, while showing
In a movie, while showing the explosion of a spacecraft in space, a flash of light is shown along with sound at the same time. What are the errors in this depiction?
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⏸▶ Pause and Ponder
- 3. Assertion (A): We cannot hear a bell in an evacuated jar.
- Reason (R): Sound requires a medium to travel.
- Both A and R are true.
- R correctly explains A — no air means no medium.
- Answer: (ii) Both A and R true, R is the correct explanation.
✅ Test Yourself
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What is a medium?
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The material through which sound propagates. -
Name the three kinds of medium sound can travel through.
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Solids, liquids and gases. -
What is a vacuum?
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A space where there is no medium (no matter) at all. -
Why can astronauts not hear each other in space?
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Space is a near vacuum, and sound cannot propagate without a medium.
Important Definitions
- Vacuum — a space that contains no matter, and therefore no medium for sound to travel through.
- Propagation — the travelling of sound outward from the source through a medium.
