Anselm stretches a long toy spring down a hallway. He flicks one end sideways, and a sideways wiggle races along it. Then he gathers a few coils at the same end and releases them, and a traveling squeeze runs down the spring instead.
Same spring, same hand, two different-looking waves. The difference is in which way the spring's own particles swing, and it sorts every mechanical wave into one of two classes.
Watch one coil during the sideways flick. The wave travels down the hallway, but the coil swings side to side, at right angles to the wave's travel direction.
A wave whose particles oscillate perpendicular to the direction the wave travels is called a transverse wave. The flicked spring carries a transverse wave.
Now watch one coil after the gather-and-release. The wave still travels down the hallway, but this time the coil swings forward and back along the hallway, in line with the wave's travel.
A wave whose particles oscillate parallel to the direction the wave travels is called a longitudinal wave. The squeezed spring carries a longitudinal wave.
The rope wave from earlier lessons is transverse: each bit of rope swings up and down while the wave travels horizontally.
Ripples cross a pond while a bit of surface water bobs straight up and down. The ripples travel horizontally. Is the ripple a transverse wave or a longitudinal wave?
Ask the sorting question: the bit of water oscillates vertically, and the wave travels horizontally.
Vertical is perpendicular to horizontal.
The particles oscillate perpendicular to the travel direction, so the ripple is a transverse wave.
A deep hum travels along a steel fence rail. Each bit of steel swings forward and back along the rail, in line with the hum's travel. Is this wave transverse or longitudinal?
Ask the sorting question: the bits of steel oscillate along the rail.
That is parallel to the travel direction.
The particles oscillate parallel to the travel direction, so the wave is longitudinal.
A wave runs along a clothesline while each bit of line swings up and down, perpendicular to the line. Is the wave transverse or longitudinal?
Beata gathers coils at one end of a spring lying straight on a gym floor and lets go. A disturbance travels its length while each coil swings forward and back, parallel to the spring. Is the wave transverse or longitudinal?
As a bass note passes down a hallway, each bit of air swings forward and back, parallel to the direction the sound travels. Is the sound wave transverse or longitudinal?
Dilys flicks a rope hanging down a flagpole. The wave travels up the rope while each bit of rope swings horizontally, perpendicular to the rope. Is the wave transverse or longitudinal?
Ripples cross a puddle while each bit of surface water bobs up and down, perpendicular to the ripples' travel. Is the ripple transverse or longitudinal?
Cosmo claps a book flat against the end of a long foam pool noodle. A squashed-together patch travels along the noodle while each bit of foam swings parallel to it. Is the wave transverse or longitudinal?
In which case is the wave longitudinal?
What decides whether a wave is transverse or longitudinal?
The snapshot shows a wave on a spring, with the marked particle's motion arrow drawn parallel to the wave's travel arrow. Which kind of wave is shown?