Section 1 of 4Waves at a Boundary
When a wave reaches a boundary between two different materials, part of the wave is usually reflected and part is transmitted.
Reflection
- Reflection Part of the wave bounces back from the boundary and travels back through the first material.
Transmission
- Transmission Part of the wave passes into the second material, usually at a different speed.
Speed change
- Speed change The speed of a wave changes when it enters a material with different properties, so the wave bends at the boundary.
Boundary strength
- Boundary strength The larger the difference between the two materials, the larger the proportion of the wave that is reflected.
Section 2 of 4Using Waves to Probe Inside
Sending a pulse into a material and timing its reflections reveals where the boundaries are.
1
A pulse of waves is sent into the object or material being investigated.
2
Waves reflected from each boundary return to a detector, and the time taken for each echo is recorded.
3
Because the wave speed in the material is known, the distance to each boundary is found from the time taken.
4
Repeating the measurement at different positions builds up a picture of the internal structure.
5
The same principle is used with ultrasound inside the body and with seismic waves inside the Earth.
| Feature | Ultrasound scan | Seismic survey |
|---|---|---|
| Source of waves | Transducer in a probe | Explosion or earthquake |
| What is probed | Soft tissue, such as a fetus | Layers of rock beneath the surface |
| Boundaries detected | Tissue and fluid boundaries | Changes in rock type or density |
Section 3 of 4Limitations of the Method
Limitations of the Method
Hidden regions
- Hidden regions Waves cannot pass easily through air or bone, so regions behind them cannot be examined.
Resolution
- Resolution Waves of higher frequency give finer detail but are absorbed more quickly, so there is a trade-off between detail and penetration.
Interpretation
- Interpretation Images must be interpreted carefully, because reflections from small features can look similar to reflections from larger boundaries.
Section 4 of 4Exam Checklist & Tips
6.1.10 – Key Points for the Exam
- Reflection The bouncing back of a wave at a boundary between two materials.
- Transmission The passing of a wave through a boundary into another material.
- Echo A reflected wave detected after it has returned from a boundary.
- Boundary The surface between two different materials, where a wave is partly reflected.
- Penetration How far a wave can travel into a material before it is absorbed.
Common Mistakes
- Saying waves always pass completely through a boundary; part of the wave is almost always reflected.
- Confusing the speed of the wave in the material with the speed of the probe or the sensor.
- Forgetting to halve the echo time when calculating the distance to a boundary, since the wave travels there and back.
- Assuming a wave can reveal structure inside an object that has no boundary to reflect from.