Describing Waves

What Is a Wave? A travelling disturbance

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

Example

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

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.

wave: energy travels, the medium does not\text{wave: energy travels, the medium does not}
A wave on a stringA wavy curve showing the shape of a string carrying a wave.

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.)

transverse: displacement⊥travel\text{transverse: displacement} \perp \text{travel}
Up-and-down while the wave goes sidewaysThe wavy string with a marked point that moves up and down.this point moves up/down

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.

snapshot: y vs space≠history: y vs time\text{snapshot: } y \text{ vs space} \quad\ne\quad \text{history: } y \text{ vs time}
Snapshot: displacement versus distanceThe wavy string frozen at one instant, its horizontal axis marking distance along the string.distance ->
waves The single most important idea: a wave picture is a snapshot in space, not in time.