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Lesson: Chapter 4 — Waves and their applications

4.3. Sound and how it travels 4 of 4

No sound in a vacuum

Unlike light, sound must have a medium to travel through. Sound waves are mechanical waves, so sound cannot cross a vacuum.

The bell-jar experiment

  • An electric bell is fixed inside a bell jar, with its connecting wires brought outside to a power supply and a switch.
  • A vacuum pump is connected to the jar, so the air inside can be removed.
  • The bell is set ringing, then the vacuum pump is switched on.

Figure to be added

Figure 4.26 — a bell jar on a table with an electric bell inside, wires to a battery and switch outside, and a pipe to a vacuum pump.

The observation

As the air is pumped out, the sound of the bell grows fainter and fainter, until in the end it cannot be heard at all — even though the bell can still be seen working.

When the sound can no longer be heard, the jar has become a vacuum. The experiment shows that sound cannot travel through a vacuum, and needs a medium.