Waves for detection and exploration (HT, physics only)

SPEC 4.6.1.5 • DETECTION & EXPLORATION

Waves For Detection & Exploration

Two specialist applications: ultrasound for medical and industrial imaging, and seismic waves to probe the Earth's interior. Both work by exploiting how waves behave at boundaries between different materials.

⚠️ HT ONLY • PHYSICS ONLY (not in Combined Science, not in Foundation tier)

The Common Principle

Both topics in this lesson are about using waves to "see" things you cannot directly observe — inside the body, inside a steel beam, under the sea, deep inside the Earth.

Spec Wording

"Differences in velocity, absorption and reflection between different types of wave in solids and liquids can be used both for detection and exploration of structures which are hidden from direct observation."

Three Things Waves Do Differently In Different Materials

Velocity — wave speed changes when the medium changes (sound: ~340 m/s in air, ~1500 m/s in water, ~5000 m/s in steel)

Absorption — some materials absorb certain waves; others let them through

Reflection — waves partially reflect at every boundary between two different media

By emitting a wave and measuring what comes back (and when), we can deduce what's inside. That's the whole game.

Ultrasound

Lesson 6.03 told you that humans can hear from 20 Hz to 20 kHz. Ultrasound is sound that's above that range.

Ultrasound

Spec wording:"Ultrasound waves have a frequency higher than the upper limit of hearing for humans."

In practice: ultrasound frequencies are above 20 kHz. Medical ultrasound is typically in the range 1–15 MHz (millions of Hz). Bats and dolphins use ultrasound to navigate naturally.

The Key Behaviour

Spec wording:"Ultrasound waves are partially reflected when they meet a boundary between two different media."

The word "partially" matters. Most of the pulse continues into the second medium; some is reflected back. By timing how long the reflected pulse takes to return, you can calculate the distance to that boundary.

The Echo Equation

A pulse leaves the probe, travels to a boundary, reflects, and returns to the probe. We measure the total round-trip time.

d = (v × t) / 2

distance (m) = speed (m/s) × time (s) ÷ 2

Why ÷ 2?

The pulse travels distance d to the boundary, then distance d back — so v × t covers 2d, not d. We halve to get the one-way distance.

Not On The Equation Sheet

The only wave equations on the AQA equation sheet are v = fλ and T = 1/f. You need to derive d = (v × t) / 2 yourself from distance = speed × time, and halve for the round trip. Show this derivation if asked to "explain" your method.

Worked Example 1 — Medical Scan

During a fetal scan, an ultrasound pulse takes 0.15 ms to travel from the probe to the fetal heart and back. The speed of ultrasound in body tissue is 1500 m/s.

Calculate the depth of the fetal heart below the probe. 3 marks

E

d = (v × t) / 2

S

Convert: t = 0.15 ms = 0.000 15 s = 1.5 × 10⁻⁴ s

d = (1500 × 0.000 15) / 2 = 0.225 / 2

U

metres (m)

A

d = 0.1125 m ≈ 0.11 m (or 11 cm)

The trap: students who forget to halve get 0.225 m (22 cm) — that's the round-trip distance, not the depth. The mark scheme rejects this even though the arithmetic is correct.

Worked Example 2 — Sonar

A ship uses echo sounding to find the depth of the seabed. The pulse returns to the ship after 0.40 s. Speed of sound in seawater = 1500 m/s.

Calculate the depth of the seabed. 3 marks

E

d = (v × t) / 2

S

t = 0.40 s (already in SI)

d = (1500 × 0.40) / 2 = 600 / 2

U

metres (m)

A

d = 300 m

Echo sounding works the same way as medical ultrasound — same equation, same logic. Only the medium and the pulse frequency are different.

Where We Use This

The same physics powers a wide range of applications — all three named explicitly in the spec.

ApplicationWhat's detectedWhy it works

Medical imaging (e.g. fetal scans, organ scans)Boundaries between tissues and organs inside the bodyPartial reflection at every tissue boundary; safe (no ionising radiation)

Industrial imaging (flaw detection in metals)Cracks or voids inside metal castings or weldsAn internal defect creates a boundary; pulse reflects sooner than from the back wall

Echo sounding (sonar)Objects in deep water; seabed depthSpec wording: "high frequency sound waves used to detect objects in deep water and measure water depth"

Why Ultrasound Specifically?

High frequency means short wavelength, which gives finer resolution — small features can be resolved. Audible-range sound has wavelengths too long for detailed imaging.

Seismic Waves

Earthquakes release huge amounts of energy that travels through the Earth as seismic waves. By recording them at the surface, geophysicists can deduce what the inside of the Earth is like — even though we've never drilled deeper than about 12 km.

Spec Wording

"Seismic waves are produced by earthquakes. P-waves and S-waves provide evidence for the structure and size of the Earth's core."

Two Types

Earthquakes generate P-waves (primary) and S-waves (secondary). They travel at different speeds and through different materials. Their differences are what makes them useful — if they behaved the same, we'd learn nothing.

P-Waves Vs S-Waves

PropertyP-wave (primary)S-wave (secondary)

Wave typeLongitudinalTransverse

Travels throughSolids AND liquids (and gases)Solids ONLY — cannot travel through a liquid

SpeedFaster — travels at different speeds through solids and liquidsSlower than P-waves

Arrival at seismometerArrives FIRST (hence "primary")Arrives SECOND (hence "secondary")

Critical Fact — Memorise

S-waves cannot travel through a liquid. This single fact is the basis of how we know the structure of the Earth's core. Spec wording verbatim.

Spec-Required Recall

From Jun 2018 P2 Q03.1 (multiple choice): "P-waves are longitudinal and S-waves are transverse." Often a 1-mark gimme — don't lose it.

Inside The Earth

The Earth has four main layers. The behaviour of P and S waves at each layer's boundary tells us what each layer is made of.

Reading The Diagram

P-waves (blue) curve through the mantle, pass into the outer core (changing speed), continue through the inner core, and exit on the other side

S-waves (red) travel through the solid mantle BUT stop at the boundary with the liquid outer core — they cannot continue

Why P-Wave Paths Curve

The spec says P-waves "travel at different speeds through solids and liquids". This speed change at each layer boundary causes the waves to refract — bend toward the new direction. That's why the P-wave paths in the diagram are curved, not straight lines.

The S-Wave Shadow Zone

Because S-waves can't pass through the liquid outer core, there's a region on the far side of the Earth from an earthquake where only P-waves arrive — no S-waves. This region is called the S-wave shadow zone.

What Each Type of Detector Sees

Detectors close to the earthquake: both P-waves and S-waves arrive (P first, then S)

Detectors in the S-wave shadow zone (on the far side, blocked by the outer core): only P-waves arrive

This is the evidence that the outer core is LIQUID — if it were solid, S-waves would pass through and reach the far side too

The Reasoning Chain Examiners Want

(1) S-waves are transverse and cannot travel through a liquid. (2) The outer core is liquid. (3) So S-waves stop at the outer core. (4) Detectors on the far side receive only P-waves (which can pass through liquid). Every link in this chain matters.

What Seismic Waves Taught Us

The spec asks you to be aware that seismic waves provided new evidence about parts of the Earth not directly observable. This is a "history of science" point.

Spec Wording

"The study of seismic waves provided new evidence that led to discoveries about parts of the Earth which are not directly observable."

Key Discoveries

The outer core is liquid — deduced from the S-wave shadow zone

The inner core is solid — deduced from P-wave behaviour (P-waves speed up again as they enter the inner core, which only happens in a solid)

The size of the core — from the angular width of the shadow zone (the bigger the core, the wider the shadow)

Why This Matters

We can't drill more than ~12 km into the Earth (the crust is 5–70 km thick; the core is ~2900 km down). Seismic waves are the only way we have to study the deep Earth. Scientific knowledge of a hidden structure, built entirely from indirect wave evidence.

Examiner Alert

From Jun 2024 P2 Q4.4

68% of students correctly identified the position where only P-waves are detected after an earthquake. But the reasoning given was often weak — vague answers like "it's blocked by the layers" or "the waves can't get there".

The correct reasoning always names which wave is blocked, where it's blocked, and why: S-waves cannot travel through a liquid; the outer core is liquid; so S-waves don't pass through it; only P-waves reach the far side.

Mark Scheme — Jun 2018 P2 Q03.5 (verbatim)

Two marks for: "because S-waves cannot travel through a liquid AND S-waves do not travel through the (outer) core". Allow 1 mark for: "some (seismic) waves cannot travel through a liquid and do not go through the core".

Weak Vs Strong — Shadow Zone Explanation

"A seismometer on the opposite side of the Earth from an earthquake records only P-waves. Explain why." (3 marks)

Weak — 0 or 1/3

Answer: "The layers block the S-waves. They can't get through to the other side."

Why it fails: Vague. Doesn't name which layer, doesn't say why it blocks S-waves (the property of liquids), doesn't mention transverse nature. Examiners reject this — see Jun 2024 examiner report.

Strong — 3/3

Answer:

"S-waves are transverse and cannot travel through a liquid. The Earth's outer core is liquid, so S-waves cannot pass through it. Detectors on the far side, in the S-wave shadow zone, therefore only receive P-waves — which CAN travel through liquid."

Three-Link Chain

(1) S-waves can't pass through liquid → (2) outer core is liquid → (3) S-waves don't reach the far side. All three links scored separately.

Common Errors

Students often writeCorrect versionWhy it matters

"d = v × t" (forgets to halve)d = (v × t) / 2v × t is round-trip distance; d is one-way

Substitutes time in ms without convertingConvert ms → s before substitutingAnswer comes out 1000× too big

"The whole core is liquid"Outer core is liquid; inner core is solidSpec-required distinction; P-wave behaviour in the inner core shows it's solid

"P-waves are transverse"P-waves are longitudinal ; S-waves are transverseReversed — common slip even in MCQs

"S-waves are slower so don't reach the far side"S-waves are slower, but the reason they don't reach the far side is that they can't pass through liquidConfuses speed with whether the wave can travel at all

"Ultrasound is electromagnetic"Ultrasound is a sound wave (mechanical, longitudinal) — just very high frequencyConflates EM and mechanical waves

Practice Question

(a) Define ultrasound. 1 mark

(b) An industrial ultrasound probe is used to detect a crack inside a steel beam. The pulse takes 8.0 μs to travel from the probe to the crack and back. Speed of sound in steel = 5000 m/s. Calculate the depth of the crack. 3 marks

(c) Compare P-waves and S-waves. 2 marks

(d) A seismometer on the opposite side of the Earth from an earthquake detects only P-waves. Explain how this provides evidence about the structure of the Earth's core. 3 marks

(a) Sound waves with a frequency above the upper limit of human hearing (above 20 kHz). [1 mark]

(b) E: d = (v × t) / 2. [1 mark]

S: Convert t = 8.0 μs = 8.0 × 10⁻⁶ s. d = (5000 × 8.0 × 10⁻⁶) / 2 = 0.040 / 2 = 0.020 m (or 2.0 cm)[1 mark substitution + 1 mark answer with unit]

(c) P-waves are longitudinal; S-waves are transverse. [1 mark] P-waves can travel through solids AND liquids; S-waves can only travel through solids (cannot travel through liquids). [1 mark]

(d) S-waves cannot travel through a liquid. [1 mark] The fact that S-waves don't reach the far side means a layer of the Earth's core must be liquid — this is the outer core. [1 mark] Only P-waves reach the far side because P-waves CAN travel through liquid. [1 mark]

Lesson Summary

Lesson 6.04 complete. This is HT and Physics only — narrow audience, but it's a regular Paper 2 topic.

d = vt/2

Echo distance

P + S

Seismic types

Liquid

Outer core

✓ Checkpoint — You Can Now…

Define ultrasound (frequency above the upper limit of human hearing)

Explain that ultrasound is partially reflected at boundaries between different media

Apply d = (v × t) / 2 for medical, industrial, and sonar/echo-sounding calculations

State three uses: medical imaging, industrial imaging, echo sounding

Distinguish P-waves (longitudinal) from S-waves (transverse)

State that S-waves cannot travel through a liquid , P-waves can

Explain the S-wave shadow zone as evidence that the outer core is liquid

Be aware that seismic waves provide evidence about parts of the Earth that are not directly observable

Next Up

Lesson 6.05 — Types of electromagnetic waves. The seven-type EM spectrum: radio, microwave, infrared, visible, UV, X-rays, gamma. All transverse, all the same speed in vacuum.