# Wave Types

## Types of Waves

What things in the natural world are 'waves'.

You might think of the sea, which creates waves we can surf on. Or a puddle of water will show ripples when a stone is dropped in it.

Water ripples are good examples of transverse waves. These are waves which move at right angles (vertical) to the direction of wave transmission (horizontal).

You might also think of 'soundwaves' or 'radiowaves'. But, is light also a wave? And if so, is it a different sort of wave to sound?

So, what are waves, how are they formed, and why are they so important to physics?

A wave is a disturbance moving through a medium. The medium can be air, liquid, or solid. The wave travels through the medium and carries energy with it.

Waves are a transmission of energy. They pass through the particles of a medium, and cause them to move up and down (transverse) or backwards and forwards (longitudinal).

Waves do not always need a medium. Some waves can also pass through vacuum. These are called electromagnetic waves, and carry with them an electrical field and a magnetic field. Light is an example, and passes through space in little packets called photons.

Since waves carry energy, they can also transmit information. By altering the frequency and amplitude of soundwaves we can create music or speech. Radio waves can carry information which electronic devices can interpret and turn into audiovisual information, so we can watch TV or listen to the radio.

## Transverse Waves

A simple slinky spring demonstrates transverse waves.

The particles of the medium which pass these waves move at right angles to the path of the wave. The particles in a spring move up and down while the wave passes forward.

An example of transverse waves are the waves on the surface of water. If you are floating on the water and a wave passes, you move up and down, while the wave passes from behind to in front of you. The water particles also move up and down - only the energy of the wave moves forward.

A transverse wave may be plotted against time. The period of the wave is the time interval between two successive wave crests passing a point.

Transverse waves can pass through solids. Earthquake seismic tremors have both compression waves and transverse waves. Compression waves travel faster, so earthquakes are typically experienced by an initial compression wave (P-wave), followed by an aftershock transverse wave (S-wave). The transverse waves can cause serious damage by dislocating large areas of the crust.

Buildings and bridges can develop resonance, a phenomenon by which reflected tansverse waves are superimposed at exactly the right frequency.

Longitudinal, or compression, waves are the types of waves which alternatively squeeze and release the medium to pass their energy. The medium is alternately compressed (compression zone) and expanded (rarefaction zone) as the wave passes through. This is why a loudspeaker diaphragm moves in and out to generate its soundwaves.

Compression and rarefactions in a longitudinal wave

A good example of a longitudinal wave is sound. Sound propagates by a pressure wave compressing a region of air, while behind it a region of less dense air is formed. Once the sound wave has passed through, the air all returns to its previous average density.

Longitudinal waves in a helical spring.

The particles which comprise the medium move backwards and forwards in line with the direction of the wave.

Earthquake damage: p-waves arrive first and s-waves second. S-waves are shear waves and move side to side.

Compression waves occur only in fluids (liquids and gases). Other examples of compression waves are sound passing through water and explosions.

## Earthquakes

Earthquakes are a release of energy within the Earth's crust, which causes a radiation of energy in all directions. This energy is carried by two types of waves: transverse and longitudinal.

In an Earthquake, the longitudinal wave is also called the P-wave (primary or pressure wave). As the pressure wave moves, the various layers of the ground act as a fluid rather than a solid structure. Because longitudinal waves travel faster than transverse waves, the P-wave is the first of the shockwaves to arrive at a place affected by the Earthquake.

The secondary wave, or S-wave, is a transverse wave (or shear wave). The S-wave is more destructive because it moves the ground side to side, putting buildings under enormous shear and torsion strains.

P-waves are detected by seismographs, and since they arrive before the S-wave can be used as a brief early warning signal.

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