Transverse & Longitudinal Waves
Transverse & Longitudinal Waves
- In a transverse wave the vibration is at right angles to the direction the wave travels.
- In a longitudinal wave the vibration is along the direction the wave travels.
The two types, side by side
- Take a long spring (a slinky) on a bench. Move the end from side to side and the wave runs along the spring while each coil moves across: that is transverse.
- Push and pull the end in and out instead, and you send squashed and stretched regions along the spring: that is longitudinal.
- The squashed regions are compressions; the stretched regions are rarefactions. In sound these are just high and low air pressure.
longitudinal:
push & pulltransverse:
side to side| Transverse (vibrate across) | Longitudinal (vibrate along) |
|---|---|
| light and all EM waves | sound |
| waves on a string or rope | compression (pressure) waves |
| ripples on water | waves along a pushed slinky |
Check the direction of vibration: across the travel direction = transverse; along it = longitudinal. Sound is the longitudinal one to remember.
Drawing a longitudinal wave as a sine
- A row of dots crowded together and spread out is difficult to measure, so we redraw the same wave as a transverse sine.
- A compression (dots crowded) becomes a peak, a rarefaction (dots spread) becomes a trough. The wavelength and amplitude are then easy to read off.
Common mistake
Sound is longitudinal. If a question shows a sound wave drawn as a sine, that sine is only a convenient picture. The air still vibrates back and forth along the wave, not across it.
Your turn— tap to reveal the worked answer (9702-style)
State the difference between a transverse wave and a longitudinal wave.
Answer: in a transverse wave the particles vibrate at right angles to the energy transfer; in a longitudinal wave they vibrate parallel to it.