K323 / 2027
General properties of waves overview

Topic 1 of 6

What travels in a wave?

A wave is a travelling disturbance that transfers energy. The material it passes through does not have to travel along with it.

An oscillation is a repeated motion about an equilibrium position. A vibrating part can move away from that position and return while passing a disturbance to its neighbours.

Follow a mark on a rope

Give one end of a stretched rope a brief up-and-down movement. A pulse travels along the rope. A small mark on the rope rises and falls as the pulse passes, then returns to its starting position.

The mark identifies the same piece of material throughout. It does not travel to the far end with the pulse. Yet the disturbance can make something at the far end move, showing that energy has been transferred.

Move the end repeatedly and a train of waves is produced. Each part oscillates about its local position as the pattern travels. This is what it means for a wave to transfer energy without transferring matter along with the disturbance. An independent bulk flow, such as a water current, is a separate effect.

A travelling disturbance and a local vibration

Green arrows show wave travel. Blue double arrows show the directions of local oscillation, not the instantaneous velocity at a turning point. Orange marks the same material point in each pair.

Transverse pulse on a rope

The rope pulse moves right while a marked part returns to its original heightTwo snapshots show one right-moving pulse. Earlier, the orange marker is at the pulse's crest. Later, the pulse is farther right and the marker has returned to the horizontal equilibrium line. The marker has the same horizontal position in both snapshots. A vertical blue double arrow indicates the directions in which the rope can vibrate, perpendicular to the horizontal green propagation arrow. The static snapshots are schematic and do not give a measured time interval.Pulse travels rightEarlierLaterThe marker stays at the same xLocal motion is up and down

Longitudinal pulse in a spring

A compression travels along a spring while a marked coil moves locally to and froTwo snapshots contain the same 51 schematic coils. The closely spaced compression moves from the left part of the spring to the right. The orange marked coil is slightly left of its dashed equilibrium reference in the earlier snapshot and back at that reference in the later snapshot. It does not follow the compression along the spring. A horizontal blue double arrow indicates local to-and-fro motion, parallel to the green propagation arrow. The models do not provide a numerical amplitude or speed.Compression travels rightEarlierLaterThe marked coil moves locallyLocal motion is along the spring
Compare the wave's direction of travel with the marked material's local motion. The double-headed arrows show the directions in which the material can oscillate, not an instantaneous velocity at a turning point. The rope pulse and the compressed spring pattern travel while their material moves locally.

Compare transverse and longitudinal motion

Transverse wave
The material vibrates perpendicular to the direction of wave travel. A wave travelling horizontally along a rope while its material moves up and down is an example. A spring can also carry a transverse disturbance if its end is moved sideways relative to its length.
Longitudinal wave
The material vibrates parallel to the direction of wave travel. Push and pull the end of a stretched spring along its length: close and wide coil spacings travel along it while a marked coil moves to and fro locally. Sound in air is another example.

The spring's close-spaced region is a compression; its more widely spaced region is a rarefaction. Those regions travel, but a particular coil does not stay in a compression and travel all the way along the spring.

Worked classification

Two disturbances travel to the right

In the first, marked points move up and down. Their vibration is perpendicular to the rightward travel, so the wave is transverse.

In the second, marked points move left and right. Their vibration is parallel to the rightward travel, so the wave is longitudinal. Calling a wave "horizontal" is not enough: both patterns travel horizontally in these examples.

Wavefronts in a ripple tank

A ripple tank is a shallow transparent tank in which disturbances on the water surface can be observed. A vibrating straight dipper can produce approximately straight wavefronts. A small point source can produce circular wavefronts spreading outwards.

A wavefront joins points at the same stage of oscillation, such as points along one crest. In the uniform region shown, the direction of wave travel is perpendicular to the local wavefront. Straight fronts travel across the tank; circular fronts spread radially away from the source.

Ripple-tank wavefronts, seen from above

Each green line joins crests at one instant. Arrows show travel perpendicular to the local front in the uniform region shown. The lines are not paths of water particles.

A straight dipper gives straight fronts

Parallel straight crests travel away from a straight dipperIn a top view of a ripple tank, a straight dipper at the left produces parallel vertical crest lines. A horizontal green arrow points right, perpendicular to those lines. The dipper vibrates to produce the waves; the drawing does not show material travelling with a crest.DipperTop view: lines of crests

A point source gives circular fronts

Circular crests travel radially outwards from a point sourceA top view shows a small orange vibrating point source at the centre of three concentric circular crests. Outward radial arrows cross the fronts at right angles. Every circle is a wavefront, not the orbit of one water particle.Orange dot: vibrating point source
These are top views of successive crests in a ripple tank. The lines represent wavefronts, not rows of water particles travelling together. The travel arrows are perpendicular to the local wavefronts.

Water surface patterns help show wavefronts and crest spacing. The rope is the simpler model for purely up-and-down transverse motion; real water-particle motion need not be an exactly vertical line.

Separate observation from explanation

Watch a marked point on a rope or spring as the disturbance passes, and compare its position before and afterwards. Describe that local motion separately from the changing position of a crest or compression. In a ripple tank, observe successive fronts and their spacing; the front itself is a pattern rather than a permanent set of particles.

Compare two directions. Transverse and longitudinal describe vibration relative to propagation. They do not mean that the material traces the whole drawn wave shape or travels with the crest.

Optional check A pulse travels to the right along a rope. A small mark on the rope rises and falls, then returns to its starting position. What has travelled along the rope?
A pulse travels to the right along a rope. A small mark on the rope rises and falls, then returns to its starting position. What has travelled along the rope?