What Is a Wave? A travelling disturbance - Waves, Oscillations, and Resonance Step by Step

A wave carries energy through a medium without carrying the medium along; its snapshot shows displacement versus distance at one instant.

waves The single most important idea: a wave picture is a snapshot in space, not in time.

Example

A wave carries energy through a medium without carrying the medium along; its snapshot shows displacement versus distance at one instant. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.

highlighted = computed this step

A wave is a travelling disturbance

A wave is a disturbance that travels through a medium, carrying energy from place to place without carrying the material along. A wave on a string is the classic picture: the string itself only moves up and down, while the wave shape moves along it.

\text{wave: energy travels, the medium does not}

Transverse: the medium moves across the travel

In a transverse wave the bits of medium move perpendicular to the direction the wave travels: the string goes up and down while the wave moves sideways. (In a longitudinal wave, like sound, the medium instead squashes and stretches along the travel direction.)

\text{transverse: displacement} \perp \text{travel}

This picture is a snapshot in space

Read this carefully: the curve is a SNAPSHOT — it shows the displacement at every place at one frozen instant. Its left-right axis is distance, not time. To see how often the wave repeats in TIME you must instead watch a single point bob up and down. Space and time are two different views of the same wave.

\text{snapshot: } y \text{ vs space} \quad\ne\quad \text{history: } y \text{ vs time}