Learning outcomes
- Distinguish transverse and longitudinal waves by the direction of vibration.
- Give syllabus examples of each type.
- Describe compressions and rarefactions.
- Interpret displacement and pressure representations without confusing them.
3.1 Transverse waves
In a transverse wave, the direction of vibration is at right angles to the direction of energy transfer. A point on a rope may move vertically while the wave travels horizontally. Electromagnetic waves are transverse, as are waves on the surface of water in the simplified O Level model and seismic S-waves.
The words crest and trough are appropriate for a transverse displacement diagram. Polarisation, although not required in detail here, is possible only for transverse waves and is evidence that electromagnetic radiation is transverse.
3.2 Longitudinal waves
In a longitudinal wave, particles vibrate parallel to the direction of energy transfer. When a spring is pushed and pulled along its length, regions where the coils are close together move along the spring. These are compressions. Regions where coils are more widely separated are rarefactions.
Sound in air and seismic P-waves are longitudinal. In sound, air molecules oscillate back and forth while regions of high and low pressure travel. Molecules collide with neighbours and pass the disturbance onward. The molecules do not travel from the loudspeaker to the listener.
3.3 Comparing diagrams
A row of dots crowded together and spread apart is a particle model of a longitudinal wave. A sinusoidal graph may also represent the pressure variation in a sound wave, but the peaks on this graph are not particles moving upward. They represent high pressure or another quantity plotted on the vertical axis.
For a longitudinal wave, one wavelength is the distance between consecutive compression centres. On a pressure–distance graph it is the distance between consecutive pressure maxima. Amplitude may be represented by the maximum particle displacement or the maximum pressure variation, depending on the graph.
3.4 Examples and classification
Electromagnetic radiation: transverse and able to travel in a vacuum. Water-surface waves: treated as transverse because the visible displacement is mainly perpendicular to travel. Seismic S-waves: transverse and cannot travel through liquids. Sound and seismic P-waves: longitudinal and involve compressions and rarefactions.
Classification must refer to vibration direction, not the shape of the drawn line. A sloping or curved line does not automatically make a wave transverse. State what vibrates and compare that direction with the direction in which energy travels.
Worked examples
Classification statement
For sound travelling to the right, air molecules move alternately right and left. Their motion is parallel to the wave direction, so sound is longitudinal.
Wavelength in a spring
Compression centres occur at 0.20 m, 0.68 m and 1.16 m. The separation is 0.48 m, so the wavelength is 0.48 m.
Practical focus
Investigation
Use a slinky. Demonstrate a transverse pulse by moving one end sideways and a longitudinal pulse by pushing it along its length. Mark one coil with tape to show that the coil oscillates locally while the disturbance travels.
Examination guidance
- Do not say transverse waves “move up and down”; specify that the vibration is perpendicular to energy transfer.
- Do not draw crests and troughs as the actual path of air molecules in sound.
- Use “compression” for high particle density or pressure and “rarefaction” for low density or pressure.
Check your understanding
- Classify an electromagnetic wave and a sound wave.
- What is the distance between adjacent compressions called?
- Why can a sine curve still be used to represent a longitudinal wave?
Answers
- Electromagnetic waves are transverse; sound waves are longitudinal.
- One wavelength.
- The curve can represent pressure or displacement as a function of position or time; it is a graph, not the physical path of particles.
