Picture a slinky spring. If you push and pull one end back and forth along its length, you see coils bunching up and spreading out as the disturbance travels down the spring. That is a mechanical model of sound in air.
- Sound needs a material medium to travel, unlike light, which rules out the electromagnetic option straight away.
- In air, molecules cannot support a sideways shearing motion the way a solid can, so a transverse vibration cannot travel through it.
- Instead, air molecules vibrate along the same line as the wave's travel direction, creating compressions and rarefactions.
- This back and forth motion parallel to travel direction is the definition of a longitudinal wave.
Since polarization only makes sense for transverse waves, and sound in air is longitudinal, options A, C and D are all ruled out, leaving longitudinal as correct.