Wave Properties for Detection and Exploration Revision Notes | GCSE AQA Higher Physics

1. Why Waves See What We Cannot

Imagine trying to find a crack inside a concrete wall or locating oil deposits kilometres underground. Your eyes cannot penetrate these materials, but waves can. This is the fundamental principle behind medical ultrasound, earthquake detection, and oil exploration.

Waves behave differently when they encounter different materials. They speed up, slow down, bounce back, or get absorbed depending on what they meet. These predictable changes in wave behaviour become our 'eyes' for seeing hidden structures.

The key insight is this: when a wave travels from one material to another, the change in its properties tells us something about both materials. By measuring these changes, we can map what lies beneath the surface.

2. Wave Velocity - The Speed Detective

Wave velocity changes dramatically between materials. Sound waves travel at about 340 m/s through air, 1500 m/s through water, but over 5000 m/s through steel. This isn't random - denser, more rigid materials generally allow waves to travel faster.

When waves cross from one material to another, they maintain their frequency but change speed. This speed change causes the wavelength to alter, and often causes the wave direction to bend - a process called refraction.

Geologists use this principle to map underground structures. Seismic waves from earthquakes or controlled explosions travel at different speeds through rock, soil, oil, and water. By timing how long waves take to reach different detection stations, they can work backwards to determine what materials the waves encountered.

3. Wave Absorption - The Energy Thief

Not all wave energy makes it through materials. Some gets absorbed and converted to heat. The amount absorbed depends on the material's properties and the wave's frequency. Soft tissues absorb ultrasound differently than bone, which is why ultrasound can image inside the body.

Medical ultrasound exploits these absorption differences. High-frequency sound waves (2-15 MHz) penetrate skin and soft tissue but are strongly absorbed by gas-filled organs like lungs. Bone absorbs so much ultrasound that it creates 'shadows' in the image.

The absorption pattern reveals internal structure. Fluid-filled cysts appear dark because they absorb little energy. Solid tumours appear brighter because they absorb more. Radiologists read these absorption patterns like a map of the body's interior.

4. Wave Reflection - The Echo Mapper

When waves hit a boundary between different materials, some energy reflects back. The amount depends on how different the materials are. A sound wave travelling from water to air reflects almost completely because air and water have very different properties.

This reflection principle powers sonar and ultrasound imaging. A pulse is sent out, and the time taken for echoes to return reveals the distance to reflecting surfaces. The strength of the echo indicates how different the materials are.

Ships use sonar to map the seabed. Sound pulses reflect strongly from the water-seabed boundary because rock is much denser than water. By measuring echo timing and strength, sonar builds a detailed picture of underwater terrain, shipwrecks, and schools of fish.

Worked example: Calculating Distance from Echo Time

A ship's sonar sends a pulse that returns after 0.4 seconds. If sound travels at 1500 m/s in seawater, how deep is the seabed?

1. Sound travels down to seabed, then back up — The pulse makes a round trip - down and back up
2. Total distance = speed × time = 1500 m/s × 0.4 s = 600 m — This is the total distance travelled by the sound
3. Depth = total distance ÷ 2 = 600 m ÷ 2 = 300 m — Divide by 2 because sound travelled down AND back up

Answer: The seabed is 300 m deep

5. Real-World Applications - Waves at Work

These wave properties combine to create powerful detection technologies. Medical ultrasound uses all three: velocity changes help focus the beam, absorption differences create contrast, and reflections from organ boundaries build the image.

Earthquake monitoring relies on seismic wave velocity changes. P-waves and S-waves travel at different speeds through the Earth's layers. By comparing arrival times at multiple stations, seismologists can locate earthquake epicentres and map the planet's internal structure.

Non-destructive testing uses ultrasound to find cracks in aircraft wings and bridge supports. Waves reflect strongly from air-filled cracks because air has very different properties from metal. This allows engineers to spot dangerous flaws without dismantling structures.

6. Exam Success - Wave Detection Questions

Exam questions on wave detection typically ask you to explain how wave properties enable detection of hidden structures. The key is to link specific wave behaviours to practical applications.

When explaining how ultrasound works, mention that different tissues have different acoustic properties. This causes waves to reflect at boundaries between organs, and the reflected waves build up an image. Always connect the physics to the practical outcome.

For seismic wave questions, remember that P-waves travel through both solids and liquids, but S-waves only travel through solids. This difference helps geologists identify liquid layers like the Earth's outer core.