Waves and oscillations

A wave carries energy from one place to another without the medium itself making the trip. Watch a floating cork as a water wave passes underneath: it bobs up and down but doesn't drift along with the wave — the water isn't being transported, just the disturbance and the energy it carries.

This is easy to miss because waves you can see, like ocean waves crashing on a shore, look like they're carrying water forward. Mostly, they're not — the water molecules trace roughly circular paths and return close to where they started, while the wave pattern itself travels much further.

Waves come in two basic shapes, depending on which way the medium moves relative to the wave's travel direction: transverse waves oscillate perpendicular to their direction of travel (like light or a shaken rope), while longitudinal waves oscillate along the same direction they travel in (like sound moving through air as compressions and rarefactions).