A student places a ringing mobile phone inside a sealed glass jar. As the air is slowly pumped out of the jar, the sound of the ringing becomes quieter and eventually cannot be heard at all, even though the phone can still be seen vibrating. Explain these observations.

Eduqas GCSE Physics — 5.1 Waves in air, fluids and solids · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Sound requires a medium to propagate.
As the air is pumped out, the number of air particles decreases, so fewer particles are available to transmit the vibration of the phone.
When almost all air is removed a near‑vacuum is formed; with no particles to carry the pressure waves, the sound cannot reach the ear.
Light, however, does not need a medium, so the phone can still be seen.

Examiner tips

  • Use the term "medium" and "particles" to show understanding of sound transmission.
  • Explain the effect of reducing particle density on wave propagation.
  • Mention that a vacuum blocks sound but not light.
  • Keep the answer concise and directly linked to the marks.

Common mistakes

  • Confusing sound with light – saying the phone cannot be seen.
  • Failing to mention the role of particle density or a vacuum.
  • Using vague phrases like "less air" without explaining the effect on sound transmission.

Mark scheme (4 marks)

  1. Sound requires a medium (material/particles) to travel through
  2. As air is removed, fewer particles are present to transmit the sound wave / vibrations
  3. When all (most) air is removed a vacuum is formed, so no particles remain to transmit the sound, and no sound is heard
  4. Light (unlike sound) does not need a medium / can travel through a vacuum, which is why the phone can still be seen

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