Transverse and Longitudinal Waves: MDCAT Physics notes
Transverse and Longitudinal Waves notes for MDCAT: particle vibration direction, crests and compressions, polarization, sound, water and seismic waves.
Two ways a medium can vibrate
When a mechanical wave passes, each particle of the medium performs simple harmonic motion about its mean position; it neither stays still nor travels with the wave. What distinguishes the two types is the direction of this vibration compared with the direction in which the wave (and its energy) travels.
Transverse waves
Particles vibrate perpendicular to the direction of propagation of the wave.
- The wave consists of crests (above the mean position) and troughs (below it).
- Examples: waves on a stretched string or rope, ripples on water, electromagnetic waves (light, radio), seismic S-waves.
- Transverse waves can be polarized: their vibrations can be restricted to a single plane, e.g. by a slit or a Polaroid sheet.
Longitudinal (compressional) waves
Particles vibrate parallel to (along) the direction of propagation of the wave.
- The wave consists of compressions (particles crowded, pressure high) and rarefactions (particles spread out, pressure low).
- Wavelength = distance between two consecutive compressions (or rarefactions).
- Examples: sound waves in air, waves along a slinky spring pushed and pulled along its length, seismic P-waves. A shock wave from an earthquake that shakes buildings back and forth in the direction of travel is longitudinal.
- Longitudinal waves cannot be polarized, because vibration along the direction of travel has no transverse plane to select.
In a longitudinal wave, both the displacement of the medium and the direction of propagation are parallel to the energy transfer. In a transverse wave, the propagation is parallel to the energy transfer but the displacement is perpendicular to it.
Comparison
| Feature | Transverse | Longitudinal |
|---|---|---|
| Particle vibration | Perpendicular to propagation | Parallel to propagation |
| Made of | Crests and troughs | Compressions and rarefactions |
| Polarization | Possible | Not possible |
| Interference, reflection, diffraction | Yes | Yes |
| Examples | String waves, light | Sound, spring compressions |
Water waves and mechanical waves
Water surface waves are a combination: water particles move in roughly circular paths, having both transverse (up–down) and longitudinal (back–forth) motion. So water waves are both transverse and longitudinal.
Mechanical waves can be of either type: transverse (on ropes) or longitudinal (sound). Electromagnetic waves are always transverse.
Common MDCAT traps
- Particles of the medium perform SHM; they do not move away with the wave.
- Only transverse waves show polarization; interference, reflection and diffraction occur for both types.
- Sound is longitudinal (compressional), not transverse.
- Water waves are both transverse and longitudinal.
- Longitudinal: displacement and propagation both parallel to energy transfer.
Quick revision
- Transverse: crests and troughs, vibration ⟂ propagation.
- Longitudinal: compressions and rarefactions, vibration ∥ propagation.
- Polarization proves light is transverse.
- Earthquake P-waves are longitudinal.