Home · IGCSE 0625 · Sound · Longitudinal waves · Practice
Practice questions · Longitudinal waves

Back and forth, never across.

Six original Cambridge-style questions. They cover what makes sound longitudinal, compressions and rarefactions, why sound needs a medium, and the trap of thinking the air travels with the sound.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
The three things to hold onto

Parallel vibration. Pressure pattern. No vacuum.

01
[2 marks]

Sound is described as a longitudinal wave. Explain what this means in terms of the motion of the particles.

  • The particles vibrate back and forth about fixed positions. ✓
  • Their vibration is parallel to (along) the direction in which the wave travels. ✓
02
[2 marks]

As a sound wave passes through air, regions of compression and rarefaction form. State what is meant by each.

  • A compression is a region where the particles are pushed together, so the pressure is higher than normal. ✓
  • A rarefaction is a region where the particles are spread apart, so the pressure is lower than normal. ✓
03
Analysis
[2 marks]

An electric bell is ringing inside a sealed glass jar. As a pump slowly removes the air from the jar, the sound gets quieter and quieter until it can no longer be heard, even though the bell is still seen striking. Explain this observation.

  • Sound needs particles of a medium to travel through. ✓
  • As the air is removed there are fewer particles to carry the sound, and in a vacuum there are none, so no sound can travel out of the jar. ✓

Light can still leave the jar (it does not need a medium), which is why the bell is still seen.

04
Analysis
[2 marks]

A student says that when someone speaks across a room, the air travels from their mouth to the listener's ear. Explain why this is not correct.

  • Each air particle only vibrates back and forth about its own fixed position. ✓
  • What travels across the room is the wave and the energy it carries, passed from particle to particle, not the air itself. ✓
05
[2 marks]

Waves can be longitudinal or transverse.

(a) State how the particle vibration in a transverse wave differs from that in a longitudinal wave. [1] (b) Give one example of a transverse wave. [1]

(a) In a transverse wave the particles vibrate at right angles (perpendicular) to the direction of travel, rather than parallel to it. ✓

(b) Any electromagnetic wave, for example light (or water ripples, or a wave on a rope). ✓

06
Analysis
[2 marks]

In many science fiction films, an explosion in outer space is shown with a loud bang. Explain why, in reality, no sound would be heard.

  • Outer space is very nearly a vacuum, with almost no particles. ✓
  • Sound is a longitudinal wave that needs particles of a medium to travel, so with no medium the sound cannot travel and nothing is heard. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.