All waves transfer energy from one place to another. What changes between wave types is the direction in which the particles oscillate. Either across the path of energy, or along it.
The Key Idea
A wave on a rope is transverse: the rope moves up and down while energy travels sideways. A wave in a slinky pushed end-on is longitudinal: the coils move back and forth in the same direction the energy travels.
SECTION 01
Transverse waves.
In a transverse wave, the particles of the medium oscillate at right angles to the direction the wave is travelling.
A transverse wave on a rope. The rope moves up and down while the wave moves sideways.
Examples in the syllabus:
Water waves on the surface of a tank
Waves on a rope or stretched string
All electromagnetic waves: light, radio, microwaves, X-rays, gamma rays
S-waves in earthquakes
SECTION 02
Longitudinal waves.
In a longitudinal wave, the particles oscillate in the same direction as the wave is travelling. The wave forms regions where particles are squashed together (compressions) and regions where they are pulled apart (rarefactions).
A longitudinal wave shown on a slinky. Compressions are bunched coils; rarefactions are stretched ones. Energy travels left to right; coils oscillate left to right.
Examples in the syllabus:
Sound waves in air, water, and solids
Waves on a slinky when pushed and pulled end-on
P-waves in earthquakes
Ultrasound used in medical imaging
The Examiner's Trap
Students frequently lose marks by saying "the water moves forward" or "the air travels to your ear." Matter does not travel with the wave. Only energy travels. The particles just vibrate around their fixed rest position.
SECTION 03
Side by side.
Here is the comparison Cambridge examiners want students to be able to make.
Feature
Transverse
Longitudinal
Oscillation direction
Perpendicular to energy transfer
Parallel to energy transfer
Shape of wave
Crests and troughs
Compressions and rarefactions
Can travel in vacuum?
Only EM waves
Never. Requires a medium.
Typical example
Light, water surface
Sound, ultrasound
Worked Example
A student watches a sound wave travel from a loudspeaker through the air. Identify whether the wave is transverse or longitudinal, and explain how the air particles move in relation to the wave.
Step 1 · Identify the wave type
Sound is a longitudinal wave. This must be stated clearly.
Step 2 · Describe the particle motion
The air particles oscillate parallel to the direction the wave travels. They move backwards and forwards along the path of the wave, creating regions of compression (high pressure) and rarefaction (low pressure).
Step 3 · Connect to energy transfer
Energy is carried from the loudspeaker to the listener even though no air particle travels the full distance. Each particle only nudges its neighbour, passing the energy along.