Expert: Kevin Johnson - 1/12/2006

Question
Hello

Happy new year!

I am studying Physics B at A Level.
Could you please explain the difference between a transverse and a longitudinal wave?
I know the definition but I find difficult to understand for example how a water wave can travel in a direction but the vibrations being perpendicular to it.

Thank you for taking the time to answer my question.

Tamar

Answer
Hello Tamar,

A transverse wave is a wave where the oscillation (the amplitude) is perpendicular to the direction the wave is propagating (where it is moving to). In a longitudinal wave the oscillation is in the direction the wave is moving.

Longitudinal waves are sometimes also called pressure waves or compressional waves, as the prime examples of pure longitudinal waves are sound waves or pressure waves.

Especially in seismology longitudinal waves are called "p-waves" (primary waves) because they travel faster than transversal waves, which arrive later, hence the latter are called "s-waves" (secondary waves). The absence of s-waves travelling through the earth's core is proof that it is liquid. Also in water (a liquid) there can only be longitudinal waves.

The prime examples of pure transversal waves are electromagnetic waves (in the vacuum), where the electric and magnetic field is perpendicular to the direction of propagation. A vibrating string is another good example.

For an illustrative example of the difference take a slinky spread across a table: If you move one end up and down you get a transversal wave. If instead you move the end back and forth along the axis of the coil you get a transversal wave.

I hope you find my answer helpful.

Peace,
Kevin