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Overview of Wave Motion

by Ron Kurtus (revised 12 August 2014)

Wave motion is a disturbance in a material or medium, where the individual parts of the material may only show periodic motion, while the waveform itself moves through the material. All waves have similar characteristics, and since all forms of wave motion follow the same laws and principles, knowing the fundamentals of wave motion is important in understanding sound, light, and other types of waves.

Questions you may have include:

This lesson will answer those questions. Useful tool: Units Conversion



Definition of wave motion

Wave motion is defined as the movement of a distortion of a material or medium, where the individual parts or elements of the material only move back-and-forth, up-and-down, or in a cyclical pattern.

It appears as if something is actually moving along the material, but in reality it is just the distortion moving, where one part influences the next.

This sounds somewhat abstract, but it can be visualized with examples.

Examples of waves

The following examples shows common transverse, compression and circular waveforms.

Transverse waves

Up-and-down motion creates transverse waves

Wave in ballgame

At the ballgame, someone in the stands may start up a "wave" by standing up and then sitting down. The people on one side then stand up and sit down, then the next people, and so one.

Everyone is still in their seats, but the wave traveled through the ballpark from one end to the other.

Rope or string

You can shake a rope, causing a wave motion. The parts of the rope only move up-and-down, but the wave moves from one end of the rope to the other. A guitar string also has this type of motion.

Vibrating string demonstrates waveform

Vibrating string demonstrates waveform

Light waves

Light or electromagnetic waves are caused by a up-and-down motion of electric and magnetic fields, thus making them transverse waves.

Longitudinal waves

Back-and-forth motion creates longitudinal waves, which consists of compression of the material, followed by a rarefaction

>>>>  Longitudinal waves cause the medium to vibrate parallel to the direction of the wave. It consists of multiple compressions and rarefactions. The rarefaction is the farthest distance apart in the longitudinal wave and the compression is the closest distance together..

Comparison of compression wave and transverse wave

Comparison of compression wave and transverse wave

Slinky

You can stretch out a Slinky along the floor and give one end a back-and-forth shove. The compression will move along the Slinky to its other end.

Sound waves

A loudspeaker cone moves back-and-forth to create a sound, which is a compression wave.

AC electricity

Electrons move back-and-forth in a wire, sending a wave of electric power through the wire. The electrons stay in their general region in AC electricity, while the flow through and out the wire in DC electricity.

Circular wave

There are cases where the material moves in a combination of transverse and compression, moving in a circular or elliptical pattern.

Water waves

Drop a stone in a pool and waves move outward. The surface of the water looks like it goes up and down, but actually the water molecules move in a circular or oval motion to form the wave.

Light waves

Although light is classified as a transverse wave, the motion of the electrical and magnetic fields may be circular instead. It is hard to tell.

Characteristics of waves

The characteristics of a waveform are wavelength, amplitude, velocity, and frequency. All periodic waveforms have these common characteristics.

There are special cases, where only one crest of the wave is seen, like the the wave at a ballgame or the sound caused by an impact or explosion. In those cases, there is still a wavelength, but there is no frequency, since the waveform is not periodic.

Wavelength

Wavelength is defined as the distance from one crest (or maximum of the wave) to the next crest or maximum.

Waveform showing wavelength and amplitude

Waveform showing wavelength and amplitude

The wavelength of an ocean wave is typically several meters. The wavelength of the electromagnetic wave used in a microwave oven is in the order of a centimeter.

Amplitude

The height of the wave is called its amplitude. Some areas consider the middle of the wave to its peak as the amplitude, while others consider peak-to-peak as the amplitude.

Amplitude relates to loudness in sound and brightness in light.

Velocity or speed

The velocity of the wave is the measurement of how fast a crest is moving from a fixed point.

For example, the velocity of water waves can be measured as their speed in a given direction with respect to the land. Also, the velocity or speed of sound waves is about 1000 feet/second, while the speed of light is 186,000 miles/second.

Velocity equals wavelength times frequency:

v = λ*f

where

Note: Units for velocity may be other multiples, such as km/s or mi/s. Wavelength units must correspond with velocity units.

Frequency

The frequency of waves is the rate the crests or peaks pass a given point. Frequency is the velocity divided by the wavelength designated as cycles (or peaks) per second. Cycles per second is also called hertz.

f = v/λ

Period

The frequency is also the reciprocal of the time between crests passing a point or the period of the vibration. With this measurement:

f = 1/T

where T is the period of the vibration in seconds.

Summary

Waves are distortions in a material that may be transverse, compression or a combination of those movements. Light, sound and AC electric waves are important waveforms. The characteristics of a waveform are wavelength, amplitude, velocity, and frequency.


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Resources and references

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Top-rated books on Physics of Waves


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