About These Two Wave Types
Transverse waves
Particles oscillate perpendicular to the direction the wave travels. Light, waves on a string, and the S-waves in earthquakes are all transverse.
In the top row above, notice how each dot moves up and down while the wave shape itself slides to the right.
Longitudinal waves
Particles oscillate parallel to the direction of travel, creating alternating compressions and rarefactions. Sound in air is the canonical example.
In the bottom row, watch the tracked particle slide left and right along the same line the wave propagates.
Same math, different geometry
Both waves share the same sinusoidal displacement - only the direction of that displacement relative to propagation changes.
Why sound is longitudinal
Air can't sustain shear forces, so only compression waves (longitudinal) can travel through it. Solids can carry both types.
Tracked particle
Use the Tracked slider to highlight one particle and see its motion clearly against the backdrop of the full wave.