A Level Physics Practice Exam

Which of the following describes the particle vibration in longitudinal waves?

1. Perpendicular to the direction of wave propagation

2. Parallel to the direction of wave propagation

3. Random orientation

4. None of the above

The correct answer is: Parallel to the direction of wave propagation

In longitudinal waves, the particles of the medium vibrate in the same direction as the wave propagates. This means that as the wave travels through a medium, such as air or a spring, the molecules or particles are displaced back and forth in a direction that is parallel to the wave's movement. This back-and-forth movement creates areas of compression and rarefaction within the medium. For example, in sound waves traveling through the air, the vibrations of air molecules occur along the direction of the sound wave. When a sound source vibrates, it causes compressions in the air in front of it and rarefactions behind it, effectively transmitting sound through the medium by means of particle collision and movement that aligns with the wave's trajectory. This is distinct from transverse waves, where the particle motion is perpendicular to the wave propagation direction. Hence, the alignment of particle vibration in longitudinal waves being parallel to the direction of wave propagation is precisely what characterizes these types of waves.