Waves carry energy, not matter

A wave is a vibration (oscillation) that travels. As it moves it transfers energy (and information) from one place to another — but it does not transfer matter.

• A duck on ripples: drop a stone in a pond and ripples spread out, but a duck (or a floating leaf) just bobs up and down on the spot — it doesn't get carried across the pond. The water doesn't travel; only the energy does.
• A Mexican wave: the "wave" sweeps round the stadium, but every person just stands up and sits down — nobody runs around the ground. Flicking a rope is the same: the rope stays in your hand while the wave races to the far end.
• Sound crossing a room moves energy to your ear, but the air molecules only vibrate on the spot.

Common mistake: a wave transfers energy and information — never matter. The water, air or rope simply oscillates in place; it is not shoved along with the wave. (If matter travelled, the duck would end up on the far bank!)

Two big families: in a transverse wave the oscillations are perpendicular (at 90°) to the direction of energy transfer; in a longitudinal wave they are parallel to it. We'll picture both next.

Transverse: oscillations at 90°

In a transverse wave the particles move perpendicular to the direction the energy travels. Examples: water waves, a wave on a rope, and all electromagnetic waves.

Which wave is this?

Longitudinal: oscillations along the line

In a longitudinal wave the particles oscillate parallel to the direction of energy transfer, creating regions where they bunch up (compressions) and spread out (rarefactions). Sound is the key example, and sound travels through solids, liquids and gases.

Picture a slinky: push one end in and out along its length and a squashed-up bit (compression) travels down the spring, followed by a stretched-out bit (rarefaction). Each coil only shuffles back and forth on the spot — exactly how air carries sound.

Transverse or longitudinal?

For each wave, tap whether the oscillations are across (transverse) or along (longitudinal) the direction of travel.

Amplitude, wavelength, frequency, period

• Amplitude — the maximum displacement of a point from its rest position (metres).
• Wavelength λ — distance of one complete wave cycle (metres).
• Frequency f — number of complete waves passing a point each second, measured in hertz (Hz).
• Period T — the time for one complete wave to pass (seconds).

Frequency and period are linked — they are reciprocals of each other:

Your turn — period

Wave speed v = f λ

All waves obey one master equation linking how fast, how often and how long they are:

The wave speed is how fast the energy is carried by the wave. Rearrange to find any quantity: f = v ÷ λ and λ = v ÷ f.

Your turn — find the wave speed

Your turn — find the frequency

Your turn — find the wavelength

Measuring waves in the lab

AQA requires you to measure the speed of waves in two set-ups:

• Waves on a string — use a signal generator and vibrator. Measure the wavelength from the standing-wave pattern and read the frequency off the generator, then use v = f λ.
• Ripple tank — a dipper makes water waves. Count waves per second for the frequency and measure the distance between crests for the wavelength; again v = f λ.

Tip: to get a more accurate wavelength, measure across several waves and divide — this reduces the percentage uncertainty.

Reflection & refraction

When a wave reaches a boundary between two materials it can be reflected, absorbed or transmitted.

• Reflection — the wave bounces back (an echo of sound; your image in a mirror).
• Refraction — the wave is transmitted but changes direction because it changes speed when it crosses into the new material.

Trolley-in-the-mud analogy: picture a trolley rolling at an angle off smooth tarmac onto muddy grass. The wheel that hits the mud first slows down while the other wheel is still on tarmac — so the trolley swings round and changes direction. A wave does the same when one side of it reaches the slower medium before the other.

Common mistake: refraction is not the wave "bending for no reason." It bends only because its speed changes at the boundary. If a wave hits the boundary head-on (along the normal) its speed still changes, but it carries straight on — no change of direction.

Sound, hearing & beyond

Sound waves are longitudinal. They make the eardrum vibrate; these vibrations pass through tiny bones to the cochlea, which converts them to electrical signals for the brain. Human hearing covers roughly 20 Hz – 20 000 Hz.

Common mistake: sound is longitudinal and needs particles (a medium) to travel through, so it cannot travel through a vacuum — that's why no one can hear an explosion in space. EM waves are different: they need no medium at all.

Physics only: Ultrasound is sound above 20 kHz. It reflects at boundaries between media, so it is used for medical imaging (e.g. scanning a foetus) and industrial cleaning. Seismic waves from earthquakes reveal Earth's structure: P-waves are longitudinal and pass through solids and liquids; S-waves are transverse and cannot pass through liquids — which is how we know the outer core is liquid.

P-waves and S-waves

Your turn — ultrasound

The electromagnetic spectrum

EM waves are transverse, and in a vacuum they all travel at the same speed — 3 × 10⁸ m/s (the speed of light). The spectrum is a continuous family arranged in order of increasing frequency and decreasing wavelength:

Common mistake: gamma rays are not "faster" than radio waves. In a vacuum all EM waves travel at exactly the same speed, 3 × 10⁸ m/s. What changes across the spectrum is the frequency and wavelength (linked by v = f λ) — not the speed.

Lower or higher frequency?

Tap an EM wave, then tap the box for its frequency relative to visible light (in the middle of the spectrum).

What each part is used for

• Radio — radio & TV broadcasting (made by oscillations / alternating current in circuits).
• Microwave — cooking food and satellite communications.
• Infrared — heating, cooking, and remote controls; also thermal imaging.
• Visible — seeing, photography, fibre-optic communication.
• Ultraviolet — security marking, sun-tanning, fluorescent lamps.
• X-rays & gamma — medical imaging (X-ray photos) and treatment (killing cancer cells).

Dangers: the higher the frequency, the more harmful. UV damages skin and eyes (ageing, skin cancer). X-rays and gamma are ionising — they can knock electrons off atoms, mutating DNA and causing cancer.

Match the use

Your turn — a radio wave

Why EM waves refract

When an EM wave crosses a boundary into a different material it changes speed. If it meets the boundary at an angle, one side of the wavefront slows before the other, so the wavefronts swing round and the wave changes direction — that's refraction.

Absorbing & emitting radiation

Every object emits and absorbs infrared radiation. A perfect black body absorbs all radiation that hits it and is also the best possible emitter.

An object's temperature stays constant when it absorbs radiation at the same rate as it emits it. If it absorbs faster than it emits, it warms up; if it emits faster, it cools down.

Earth link: this balance controls the temperature of the Earth. Changes to the gases in the atmosphere alter how much radiation escapes back to space, affecting global temperatures (climate change).

Staying at one temperature

The key facts to know

You've covered AQA 4.6 — waves in air/fluids/solids, the wave equation & required practical, sound, ultrasound & seismic waves, the electromagnetic spectrum, and black-body radiation. Press Finish to see your score.