Waves

Overview

Welcome to Topic 4: Waves. This topic is a cornerstone of Combined Science, exploring how energy and information travel across the universe without the transfer of matter. From the light hitting your eyes to the sound of your favourite podcast (listen below!), waves are everywhere.

Understanding waves is crucial because it bridges the gap between simple motion and complex phenomena like the electromagnetic spectrum and radioactivity. Examiners love testing this topic because it combines visual interpretation (reading wave diagrams) with mathematical skills (applying the wave speed equation). You can expect questions asking you to label diagrams, calculate frequencies, and explain what happens when a wave crosses a boundary.

Key Concepts

Concept 1: The Nature of Waves

A wave is a disturbance that transfers energy and information from one place to another. The most important rule to remember for your exam is that waves do not transfer matter.

Think of a buoy floating on the sea. As a water wave passes, the buoy bobs up and down, but it doesn't travel towards the shore with the wave. The water particles are just oscillating (vibrating) around a fixed equilibrium position. It is the energy that travels towards the shore.

Concept 2: Transverse vs Longitudinal Waves

Waves are categorised into two main types based on how their particles oscillate relative to the direction of energy transfer.

Transverse WavesIn a transverse wave, the particles oscillate perpendicular (at right angles) to the direction of energy transfer. If the wave moves horizontally, the particles move vertically up and down.

• Examples: Light waves, all electromagnetic waves, ripples on water, seismic S-waves.

Longitudinal WavesIn a longitudinal wave, the particles oscillate parallel (in the same direction) to the direction of energy transfer. This creates regions where particles are squashed together (compressions) and regions where they are spread apart (rarefactions).

• Examples: Sound waves, ultrasound, seismic P-waves.

Concept 3: Wave Behaviour at Boundaries

When a wave reaches a boundary between two different materials (media), several things can happen:

1. Absorption: The wave's energy is taken in by the material (e.g., curtains absorbing sound).
2. Transmission: The wave passes through the material (e.g., light through a window).
3. Reflection: The wave bounces back (e.g., an echo or a mirror).
4. Refraction: The wave changes speed and direction as it crosses the boundary.

Crucial Exam Point on Refraction: When a wave refracts, its speed changes because the new medium has a different density. Because the speed changes, the wavelength must also change. However, the frequency remains constant. The source of the wave hasn't changed, so the number of waves produced per second stays the same.

Mathematical/Scientific Relationships

The Wave Speed Equation (Must Memorise)

v = f \times \lambda

• v = wave speed in metres per second (m/s)
• f = frequency in hertz (Hz)
• \lambda = wavelength in metres (m)

This equation links the speed of the wave to how many waves pass per second and how long each wave is. You must be able to rearrange this equation.

• To find frequency: f = \frac{v}{\lambda}
• To find wavelength: \lambda = \frac{v}{f}

The Distance-Time Equation (Must Memorise)

v = \frac{x}{t}

• v = wave speed in metres per second (m/s)
• x = distance in metres (m)
• t = time in seconds (s)

Use this equation when you are given the total distance a wave has travelled (like a sound wave echoing off a wall) and the time it took.

Period and Frequency (Must Memorise)

T = \frac{1}{f}

• T = period in seconds (s)
• f = frequency in hertz (Hz)

Practical Applications

Understanding waves allows us to use ultrasound for medical imaging (longitudinal waves reflecting off boundaries inside the body) and to design earthquake-resistant buildings by studying seismic P-waves and S-waves.