Waves: Transverse, Longitudinal and Key Properties

Waves are all around us, from the sound of a voice travelling through the air to the light that allows us to see the world, and even the ripples that spread across a pond when a stone is dropped into it. Understanding waves is central to physics because so many natural phenomena, and much of modern technology, depend on how waves behave. This note explores what waves are, the different types of waves, and their key properties.

What Is a Wave?

A wave is a disturbance that transfers energy from one point to another without permanently transferring matter. As a wave passes through a medium, such as water, air, or a solid material, the particles of that medium vibrate about their fixed positions but do not travel along with the wave itself. It is the energy, not the matter, that moves from place to place.

Types of Waves Based on Medium

Waves can be classified into two broad categories based on whether they need a material medium to travel.

• Mechanical waves require a medium, such as air, water, or a solid, to travel through. Sound waves and water waves are examples of mechanical waves.
• Electromagnetic waves do not require a medium and can travel through a vacuum. Light, radio waves, and X-rays are all examples of electromagnetic waves.

Transverse and Longitudinal Waves

Waves can also be classified based on the direction in which the particles of the medium vibrate relative to the direction the wave travels.

• Transverse waves are waves in which the particles of the medium vibrate at right angles (perpendicular) to the direction of wave travel. Light waves and waves on a string are examples of transverse waves. These waves have crests (the highest points) and troughs (the lowest points).
• Longitudinal waves are waves in which the particles of the medium vibrate parallel to the direction of wave travel. Sound waves are the most common example of longitudinal waves. These waves have compressions (regions where particles are close together) and rarefactions (regions where particles are spread apart).

Key Properties of Waves

Several important terms are used to describe and measure waves.

• Wavelength: The distance between two successive crests, or two successive troughs, of a wave. It is usually measured in metres and represented by the Greek letter lambda (λ).
• Amplitude: The maximum displacement of a particle in the medium from its rest position. Amplitude is related to the energy carried by the wave; a larger amplitude generally means more energy.
• Frequency: The number of complete waves produced, or passing a given point, in one second. Frequency is measured in Hertz (Hz).
• Period: The time taken for one complete wave to be produced. Period is the reciprocal of frequency.
• Wave speed: The distance travelled by a wave in one second. Wave speed is related to frequency and wavelength by the formula: speed = frequency × wavelength (v = fλ).

Wave Behaviour: Reflection, Refraction, and Diffraction

Waves exhibit certain behaviours when they interact with different surfaces, media, or obstacles.

• Reflection occurs when a wave bounces back after hitting a surface, such as an echo produced when sound reflects off a wall.
• Refraction occurs when a wave bends as it passes from one medium into another of different density, such as light bending as it passes from air into water.
• Diffraction occurs when a wave bends around obstacles or spreads out after passing through a narrow gap.
• Interference occurs when two or more waves combine, either reinforcing each other (constructive interference) or cancelling each other out (destructive interference).

Sound Waves

Sound is a longitudinal, mechanical wave produced by vibrating objects. Sound waves need a medium to travel through and cannot travel through a vacuum, which is why astronauts in space cannot hear sound directly through the vacuum of space. The pitch of a sound is determined by its frequency, while the loudness of a sound is determined by its amplitude. The speed of sound in air is approximately 340 metres per second at room temperature, though it travels faster through liquids and solids than through gases.

Light Waves

Light is a transverse, electromagnetic wave that can travel through a vacuum, which is how sunlight reaches the Earth across the vast emptiness of space. Light travels extremely fast, at approximately 3 × 10⁸ metres per second in a vacuum. Light is part of a broader family of electromagnetic waves known as the electromagnetic spectrum, which also includes radio waves, microwaves, infrared radiation, ultraviolet radiation, X-rays, and gamma rays, each differing mainly in wavelength and frequency.

Everyday Applications of Waves

Waves have countless applications in daily life and technology. Radio and television broadcasting rely on electromagnetic waves to transmit signals across long distances. Mobile phones use radio waves to communicate. Doctors use ultrasound (high-frequency sound waves) to view images of unborn babies, and X-rays to examine bones. Musical instruments produce sound waves of different frequencies and amplitudes to create the notes and tones we enjoy in music.

The Electromagnetic Spectrum in More Detail

The electromagnetic spectrum arranges all electromagnetic waves according to their wavelength and frequency. At one end are radio waves, which have the longest wavelengths and lowest frequencies, used for broadcasting and communication. Moving along the spectrum, we find microwaves, used in cooking and satellite communication, followed by infrared radiation, which we feel as heat. Visible light, the small portion of the spectrum that our eyes can detect, comes next, followed by ultraviolet radiation, which can cause sunburn, then X-rays, used in medical imaging, and finally gamma rays, which have the shortest wavelengths and highest frequencies and are produced by radioactive decay and nuclear reactions.

Resonance and Standing Waves

Resonance occurs when an object is made to vibrate at its natural frequency by an external periodic force, causing the amplitude of vibration to increase dramatically. This principle explains why a singer can shatter a glass by singing a note that matches the glass's natural frequency, and it is also used in the design of musical instruments to amplify sound. A standing wave, sometimes called a stationary wave, is formed when two identical waves travel in opposite directions and combine, producing points called nodes, where there is no movement, and points called antinodes, where movement is at its maximum.

Key Terms to Remember

• Wave: A disturbance that transfers energy from one place to another without transferring matter.
• Transverse wave: A wave in which particles vibrate perpendicular to the direction of travel.
• Longitudinal wave: A wave in which particles vibrate parallel to the direction of travel.
• Wavelength: The distance between two successive crests or troughs.
• Amplitude: The maximum displacement of a particle from its rest position.
• Frequency: The number of complete waves passing a point per second, measured in Hertz.
• Reflection, refraction, and diffraction: Key behaviours describing how waves interact with surfaces, media, and obstacles.

Summary

Waves are disturbances that carry energy from one location to another without transporting matter itself. They can be mechanical, requiring a medium, or electromagnetic, capable of travelling through a vacuum. Waves are also classified as transverse or longitudinal based on the direction of particle vibration relative to wave motion. Important properties of waves include wavelength, amplitude, frequency, period, and speed, all of which are connected by the wave equation v = fλ. Waves display characteristic behaviours such as reflection, refraction, diffraction, and interference. From the sound that lets us communicate, to the light that lets us see, to the technologies that connect our modern world, waves play an essential role in both nature and human innovation.