Introduction

Waves are everywhere.

When you hear music, see light, watch ripples move across water, or use a mobile phone, you are observing the effects of waves.

Although waves can look very different from one another, they share an important property:

A wave transfers energy from one place to another without causing an overall transfer of matter.

Understanding waves allows us to explain sound, light, earthquakes, communication technology, ocean motion, and many other phenomena.

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What Is a Wave?

A wave is a disturbance that transfers energy from one place to another.

A disturbance is a change that causes particles or fields to move or oscillate.

For example, dropping a stone into still water creates a disturbance.

Ripples then travel outward from the point where the stone entered the water.

The ripples carry energy away from the disturbance.

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Waves Transfer Energy

One of the most important ideas about waves is that they transfer energy.

Imagine a floating leaf on the surface of a pond.

As a water wave passes:

• The leaf moves up and down.
• The wave continues travelling forward.
• The leaf remains approximately in the same location.

The water itself has not travelled across the pond with the wave.

Instead, energy has travelled through the water.

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Energy Transfer vs Matter Transfer

Consider people performing a "stadium wave."

Each person:

• Stands up.
• Sits back down.
• Remains in approximately the same location.

However, the wave travels around the stadium.

This demonstrates an important principle:

The disturbance travels, but the particles do not travel with it overall.

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Particles in a Wave

In many waves, particles oscillate around a fixed or equilibrium position.

To oscillate means to move repeatedly back and forth.

For example, in a water wave, particles may move:

• Up
• Down
• Forward
• Backward

However, after the wave passes, the particles remain close to where they started.

The energy, rather than the matter, has been transferred.

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What Creates a Wave?

Every wave requires a source of disturbance.

Something must initially provide energy to create the wave.

Examples include:

Without a disturbance, there is no wave.

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Waves from Vibrations

Many waves begin with a vibration.

A vibration is a repeated back-and-forth movement.

For example, when a guitar string is plucked:

1. The string vibrates.
2. The vibrating string disturbs the surrounding air.
3. The disturbance travels through the air as a sound wave.
4. The wave reaches your ear.
5. Your brain interprets the signal as sound.

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Two Major Categories of Waves

Waves can be divided into two broad categories:

Mechanical Waves

Mechanical waves require a medium through which to travel.

Examples include:

• Sound waves.
• Water waves.
• Waves on strings.
• Seismic waves.

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Electromagnetic Waves

Electromagnetic waves do not require a medium.

They can travel through empty space.

Examples include:

• Radio waves.
• Microwaves.
• Infrared radiation.
• Visible light.
• Ultraviolet radiation.
• X-rays.
• Gamma rays.

This is why sunlight can travel through the vacuum of space from the Sun to Earth.

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What Is a Medium?

A medium is the substance through which a mechanical wave travels.

A medium can be:

• Solid
• Liquid
• Gas

For example:

Sound can travel through:

• Air.
• Water.
• Metal.
• Wood.

However, sound cannot travel through a vacuum because there are no particles available to carry the vibration.

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Transverse Waves

Waves can also be classified according to how the particles move relative to the direction of the wave.

In a transverse wave, the oscillations are perpendicular to the direction the wave travels.

For example, if a wave travels horizontally:

→

the particles may move:

↑↓

Examples include:

• Waves on a rope.
• Electromagnetic waves.
• Some seismic waves.

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Longitudinal Waves

In a longitudinal wave, particles oscillate parallel to the direction the wave travels.

Instead of crests and troughs, longitudinal waves contain:

• Compressions – regions where particles are close together.
• Rarefactions – regions where particles are farther apart.

Sound waves travelling through air are an important example.

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Comparing Transverse and Longitudinal Waves

Both types transfer energy without an overall transfer of matter.

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Common Examples of Waves

Waves appear in many situations.

Water Waves

Transfer energy across the surface of water.

Sound Waves

Carry vibrations through materials such as air.

Light Waves

Electromagnetic waves that allow us to see.

Radio Waves

Carry information used in communication.

Seismic Waves

Travel through Earth during earthquakes.

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Waves in Everyday Technology

Modern technology depends heavily on waves.

For example:

• Mobile phones use radio and microwave frequencies.
• Wi-Fi uses radio-frequency electromagnetic waves.
• Radios receive radio waves.
• Microwave ovens use microwaves.
• Medical imaging can use ultrasound, X-rays, and radio waves.
• Remote controls often use infrared radiation.
• Fibre-optic cables transmit information using light.

Without our understanding of waves, modern communication technology would not exist.

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Real-World Applications

Knowledge of waves is important in:

• Telecommunications.
• Medicine.
• Music and acoustics.
• Astronomy.
• Earthquake monitoring.
• Radar.
• Sonar.
• Wireless networking.
• Satellite communication.

The study of waves connects many areas of physics and technology.

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Worked Examples

Example 1

What does a wave transfer?

Answer:

A wave transfers energy from one place to another.

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Example 2

Does a water wave carry water across an entire ocean?

Answer:

No. Water particles mainly oscillate around their positions while the wave energy travels through the water.

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Example 3

What creates a sound wave?

Answer:

A vibrating object creates a disturbance in a medium, producing a sound wave.

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Example 4

Which type of wave requires a medium?

Answer:

A mechanical wave.

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Example 5

Is light a mechanical or electromagnetic wave?

Answer:

Light is an electromagnetic wave.

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Example 6

Is sound usually transverse or longitudinal?

Answer:

Sound travelling through air is a longitudinal wave.

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Did You Know?

Light from the Sun takes about 8 minutes and 20 seconds to reach Earth.

Because light is an electromagnetic wave, it can travel through the vacuum of space. Sound cannot do this, which means that despite what is sometimes shown in science-fiction movies, an explosion in space would be silent to a distant observer unless its effects reached them through matter.

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Key Terms

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Key Takeaways

• A wave is a disturbance that transfers energy from one place to another.
• Waves transfer energy without producing an overall transfer of matter.
• Every wave requires an initial source of disturbance.
• Mechanical waves require a medium, while electromagnetic waves can travel through a vacuum.
• Waves can also be classified as transverse or longitudinal, depending on the direction of oscillation.
• Water waves, sound, light, radio waves, and seismic waves are all examples of waves.
• Understanding waves is essential for explaining many natural phenomena and technologies, from earthquakes and music to mobile phones and satellite communication.