AQA Physics (8463) 4.6.1.5 · Investigation
Waves for detection and exploration
Doctors watch a baby before it is born, and geologists map a core that no one will ever reach. How can anyone see a structure that is hidden from view?
Triple Science · Higher tier
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Nobody has ever seen the Earth’s core.
It lies thousands of kilometres beneath your feet, and no drill has come anywhere near it. Yet geologists can tell you how big the core is, and that part of it is liquid.
If you had to guess, does their evidence come from lava, or from earthquake waves?
Earthquakes produce seismic waves that travel right through the Earth. There are two kinds. P-waves are longitudinal: they travel through solids and through liquids, at different speeds in each, and arrive first. S-waves are transverse: they travel through solids but cannot travel through a liquid. Seismometers all round the world record which waves arrive, and when. Use those records to work out what is inside.
Read the Earth · step 1 of 4
Test a solid Earth
Suppose the Earth were solid rock all the way through. Which seismometers would record S-waves from this earthquake?
No one can drill to the core or look at it. The study of seismic waves gave scientists new evidence about parts of the Earth that cannot be observed directly: the liquid outer core and the size of the core were both worked out from records like these.
Sort the evidence
P-wave, S-wave, or both?
Decide which kind of seismic wave each statement describes.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Ultrasound is sound with a frequency above the upper limit of human hearing, 20 kHz. To scan an unborn baby, a probe held on the skin sends short pulses of ultrasound into the body and listens for echoes. On the way to the baby, the pulse meets several boundaries where one kind of tissue meets another.
The scan · step 1 of 2
One pulse, three boundaries
The pulse will meet three boundaries on its way in. How many echoes come back to the probe?
Ultrasound is not ionising, so unlike X-rays it can safely scan an unborn baby. The same echo timing finds hidden cracks inside metal parts: industrial imaging. At sea, echo sounding uses pulses of high-frequency sound to measure the depth of water and to detect objects in deep water, such as shoals of fish or a wreck. Every time, the pulse goes there and back, so the boundary is half the distance travelled away.
Equations
s = v t
distance travelled (s, metres) = speed (v, metres per second) × time (t, seconds)
v = s ÷ t · t = s ÷ v
For an echo
The time is for the trip there and back. Depth of the boundary = distance travelled ÷ 2. This is a method step, not a separate equation.
Units first
Echo times are often given in milliseconds. 1 ms = 0.001 s: divide by 1000 before you insert it.
Command words in this lesson
- Explain
- Link a property of the wave to what it reveals, with "so" or "because" between them.
- Suggest
- Apply what you know to a situation you have not met. More than one answer can earn the mark.
- Use the information
- Take the pattern or the numbers from the diagram or trace you are given, and quote them.
- Calculate
- Show every line. Convert ms to s first, and halve for an echo.
Key fact
S-waves cannot cross the liquid outer core; P-waves can, but refract. An echo's time gives a boundary's depth: depth = speed × time ÷ 2.
Key note · AQA 4.6.1.5 (8463)
Waves for detection and exploration
- P-waves: longitudinal, solid + liquid.
- S-waves: transverse, solid only.
- S-waves blocked by outer core → outer core = liquid.
- Ultrasound (above 20 kHz) is partially reflected at boundaries; the echo times give their depths: medical and industrial imaging.
- Echo sounding: d = vt/2.
- Refraction at boundaries: wave changes speed → changes direction.
Photograph this card. Tomorrow, cover it and say each line first.
Previous and next
Connects to
Ask MrBadmus
Not sure how a wave can show you something hidden?
The Earth's layers are drawn to scale, but the wave paths are smooth curves and straight segments drawn to show the idea, and the shadow-zone edges (about 104° and 140°) are rounded figures. The scan's layers are simplified and every tissue is given the same speed, 1540 m/s; real tissues differ slightly, and real echoes are far weaker than the pulse.