Transverse and Longitudinal Waves

This lesson explains Transverse and Longitudinal Waves for Cambridge IGCSE Physics 0625. It covers the Core knowledge required by every candidate. Focus on the cause-and-effect explanation and the exact quantities being compared. Read the explanation once, then attempt the worked method from a blank page.

What is the difference between transverse and longitudinal waves?

The difference is the direction of vibration compared with the direction of travel. In a transverse wave, the vibrations are at right angles (perpendicular) to the direction the wave travels. In a longitudinal wave, the vibrations are parallel to the direction the wave travels. Those two sentences, with the words perpendicular and parallel, are the definition marks.

Feature	Transverse	Longitudinal
Vibration direction	Perpendicular to travel	Parallel to travel
Pattern	Crests and troughs	Compressions and rarefactions
Examples	Light, all electromagnetic waves, water surface waves	Sound
Can travel through vacuum?	EM waves: yes	Sound: no

A compression is a region where particles are squashed together; a rarefaction is a region where they are spread apart. In a longitudinal wave, one wavelength is the distance from one compression to the next compression.

Which examples does Cambridge expect you to know?

Transverse: all electromagnetic waves (light, radio, X-rays), water surface waves, and waves on a rope or spring shaken side to side. Longitudinal: sound waves, and a spring pushed and pulled along its length. A stretched slinky demonstrates both (shake it sideways for transverse, push it end-on for longitudinal), and that demonstration is a standard describe question. Note that sound needs a medium, but electromagnetic waves cross a vacuum. Classification questions often hinge on that fact.

Worked example

A loudspeaker produces a sound wave in air. (a) State whether the sound wave is transverse or longitudinal, and explain your answer. [2] (b) On a diagram of the wave, a student labels the distance from one compression to the next. State the name of this distance. [1]

Model answer: (a) Longitudinal. The air particles vibrate parallel to the direction in which the wave travels (backwards and forwards along the direction of travel). (b) One wavelength.

Original marking points:

• B1: longitudinal
• B1: vibrations/oscillations parallel to the direction of travel (allow “same direction as energy transfer”)
• B1: wavelength

These marking points belong to this original example. They are not an official Cambridge mark scheme.

Common errors and how to correct them

• Writing “up and down” and “side to side” instead of perpendicular and parallel. The mark scheme wants direction relative to wave travel, not absolute directions.
• Classifying water waves as longitudinal because water is a fluid like air. Water surface waves are transverse: the surface moves up and down while the wave travels horizontally.
• Drawing a longitudinal wave as a transverse wiggle. Sketch it as bands of close and spaced lines, labelled compression and rarefaction.
• Saying particles travel along the wave. Particles oscillate about fixed positions; only energy moves along.
• Measuring wavelength from a compression to the nearest rarefaction. That is half a wavelength.

How to apply this in an exam

For any “state and explain” classification, use a two-part template: name the type, then give the vibration direction relative to travel. One sentence each. Adding extra detail wastes time and risks contradicting yourself. Cambridge applies a “list rule” where a wrong extra statement can cancel a correct one.

Where this skill matters