Waves carry energy, not matter

Edexcel's very first point: waves transfer energy and information without transferring matter. The water in the sea, or the air carrying a sound, just oscillates about a fixed point — it does not travel along with the wave.

Evidence it's the wave that travels: a floating duck on a pond bobs up and down on the spot as ripples pass — it isn't carried to the shore. With sound, the air molecules vibrate back and forth but stay put.

What does a wave transfer?

Transverse vs longitudinal

Waves are sorted by the direction of their oscillations relative to the direction the energy travels:

• Transverse — the oscillations are perpendicular (at 90°) to the direction of energy transfer. Examples: water waves, all electromagnetic waves, and S-waves (seismic).
• Longitudinal — the oscillations are parallel to the direction of energy transfer, making compressions and rarefactions. Examples: sound and P-waves (seismic).

Transverse or longitudinal?

Tap the correct type for each wave.

Amplitude, wavelength, frequency, period

Every wave can be described by the same handful of quantities:

• Amplitude — the maximum displacement from the undisturbed (rest) position.
• Wavelength (λ) — the distance from one point on a wave to the same point on the next (peak to peak), in metres.
• Frequency (f) — the number of waves passing a point each second, in hertz (Hz).
• Period (T) — the time for one whole wave to pass a point, in seconds.
• Wave velocity — the speed at which energy is transferred; a wavefront joins points at the same stage of their cycle.

Frequency and period are linked: they are inverses of each other — f = 1/T. A 5 Hz wave has a period of 1/5 = 0.2 s.

Your turn — frequency from period

Wave speed = frequency × wavelength

This is the single most important equation in Topic 4. It works for all waves:

To find f or λ, just rearrange: f = v ÷ λ  and  λ = v ÷ f.

Misconception: a wave's speed is set by the medium it travels through, not by its frequency. If you raise the frequency in the same medium, the speed stays the same and the wavelength shrinks to compensate.

Your turn — wave speed

Your turn — rearrange for wavelength

Measuring wave speed

Edexcel's second wave-speed equation lets you measure speed from a distance and a time:

The Core Practical is to investigate equipment for measuring the speed, frequency and wavelength of a wave in a solid and in a fluid:

• Ripples on water (a fluid) — use a ripple tank. A motor makes ripples; freeze them with a strobe to measure the wavelength, and count ripples per second for the frequency. Then v = f × λ.
• Waves on a stretched string (a solid) — a vibration generator sets up a standing wave; measure the wavelength from the string and read the frequency off the signal generator.
• Sound in air — measure how long a sound takes to travel a known distance (e.g. an echo) and use v = x ÷ t.

Your turn — speed from distance & time

What happens at an interface

When a wave meets the boundary between two materials, four things can happen (and usually several at once):

• Reflection — the wave bounces back (angle of incidence = angle of reflection).
• Refraction — the wave passes through but changes speed, which can bend it.
• Transmission — the wave passes into the new material and carries on.
• Absorption — the new material takes in the wave's energy.

How much a substance absorbs, transmits, refracts or reflects a wave can vary with the wavelength of the wave.

Refraction is a change of speed

Refraction is the change of speed — and therefore often direction — of a wave as it crosses into a new medium. The frequency stays the same; because v = f × λ, a change in speed means the wavelength changes too.

Picture the wavefronts. As they enter a slower medium at an angle, the edge that enters first slows first, so the front swings round and bends towards the normal:

Why does a wave refract?

Sound, the ear & the audible range

Sound is a longitudinal wave — a series of compressions and rarefactions passing through a medium. It needs particles to travel, so it cannot travel through a vacuum.

Sound is detected when these vibrations are converted into vibrations in solids and back again — that is how a microphone, a loudspeaker and the human ear work. The eardrum vibrates, passing the motion through the tiny bones to the cochlea.

These conversion processes only work over a limited frequency range, which is why human hearing has limits. The human audible range is 20 Hz to 20 000 Hz (20 kHz).

The audible range

Ultrasound & infrasound

Ultrasound is sound with a frequency greater than 20 000 Hz — above human hearing. Infrasound is sound with a frequency less than 20 Hz — below human hearing. Edexcel expects you to explain their uses:

• Sonar / echo sounding — a ship sends an ultrasound pulse downward; it reflects off the seabed and the echo time gives the depth.
• Foetal scanning — ultrasound pulses reflect off boundaries inside the body and are used to build an image of a foetus (no harmful radiation).
• Exploration of the Earth's core — low-frequency seismic and infrasound waves travel through the Earth and reveal its internal structure (covered next).
• Infrasound & seismic monitoring — infrasound from earthquakes and volcanoes travels huge distances, so sensitive detectors can monitor distant events.

Your turn — echo sounding

Seismic waves & the Earth's core

Earthquakes produce seismic waves that travel through the Earth. Studying how they reach detectors around the world reveals the Earth's internal structure:

• P-waves are longitudinal. They travel through both solids and liquids, so they pass right through the molten outer core.
• S-waves are transverse. They travel through solids only, so they are stopped by the liquid outer core.

Because S-waves cannot cross the liquid core, there is a region on the far side of the Earth where no S-waves arrive — the S-wave shadow zone. This is the key evidence that the outer core is liquid.

P-waves and S-waves

Tap a wave on the left, then its matching property on the right.

The key facts to know

You've covered the whole of Edexcel Topic 4 — Waves. Press Finish to see your score.