Transverse and longitudinal waves

What you will learn

Compare oscillation directions, compressions and rarefactions; shared wave behaviour.

Specification mapping: A1.2

Prerequisites: A1.1.1, A1.1.2

Physical principles

In a transverse wave, displacement is perpendicular to propagation. A wave on a stretched string is a mechanical example. Electromagnetic waves are transverse oscillations of electric and magnetic fields, so no material medium is required in a vacuum.

In a longitudinal wave, displacement is parallel to propagation. Sound in air consists of pressure and density variations: compressions are regions of higher pressure and rarefactions of lower pressure. Air molecules vibrate back and forth; they do not accompany the sound all the way from speaker to listener.

Both types have frequency, period, wavelength, amplitude and speed, and both can transfer energy and undergo superposition. On a longitudinal diagram, wavelength can be measured between adjacent compression centres. A plotted pressure sine curve is a representation of pressure variation, not a picture showing air moving sideways. Only transverse waves can be plane polarised.

Scientific diagram and interpretation

Worked examples and applications

Common misconceptions

A sine-shaped pressure graph does not make sound transverse.

Examination technique

State both directions in a comparison; naming a direction without the propagation reference is incomplete.

For an explanation, connect the physical cause to its effect. For a calculation, state the model, rearrange before substitution, retain intermediate digits, and finish with units. For a data question, refer to the actual readings and distinguish a trend from proof of a cause.

Check your understanding

Five questions. Choose an option, then Check Answer. Read the feedback and use Try Again to reset. Use Tab between controls, arrow keys within radio groups, and Enter or Space on buttons. These local practice answers are not saved.