Cosmo sends a wave down a slinky on a hallway floor and watches the coils as it passes: in some stretches the coils bunch tightly together, and in the stretches between they spread wide apart.
A longitudinal wave has no crests or troughs to point at. Its landmarks are the bunched and spread stretches, and this lesson names them.
In a longitudinal wave, the medium's particles oscillate parallel to the travel direction, so the pattern shows up as spacing.
Compressions and rarefactions alternate along the wave: compression, rarefaction, compression, rarefaction.
The regions travel. Watch one bunched stretch, and it moves steadily along the slinky while each coil only swings about its own spot.
Compression and rarefaction are the longitudinal partners of crest and trough: the places where the wave shows itself most strongly.
Every longitudinal-wave lesson from here on reads these regions off the spacing of the particles.
A snapshot of a coiled telephone cord shows a stretch where the coils sit tightly bunched. What is that region?
The coils there are squeezed together.
A region where the particles are squeezed together is a compression.
The bunched stretch is a compression.
The same snapshot shows the next stretch along, where the coils sit widely spread. What is that region?
The coils there are spread apart.
A region where the particles are spread apart is a rarefaction.
The spread stretch is a rarefaction.
In a snapshot of a coiled cord carrying a wave, region J's coils sit closer together than anywhere nearby. Is region J a compression or a rarefaction?
In a snapshot of a screen-door spring carrying a wave, region K's coils sit farther apart than anywhere nearby. Is region K a compression or a rarefaction?
Anselm watches one tightly bunched stretch on a long spring as the wave runs. He says: 'This squeezed region travels along the spring with the wave.' Is Anselm correct?
Which statement defines a compression?
A snapshot of a long spring marks four regions: J with tightly bunched coils, K with evenly spaced coils, L with widely spread coils, and M with tightly bunched coils again. Which letter marks a rarefaction?
A deep rumble travels along a steel guardrail, and regions where the steel's particles crowd together pass down the rail one after another. What are these crowded regions?