Explain how the refraction of seismic waves as they travel through the Earth provides evidence for the internal structure of the Earth.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When an earthquake occurs, seismic waves travel outwards from the focus. Seismometers around the world record the arrival of these waves, but scientists notice that waves do not always travel in straight lines through the Earth.
Model answer (4 marks)
Seismic waves travel at different speeds in the crust, mantle and core because the density and elastic properties of these layers differ.
When a wave passes from one layer into another with a different speed it is refracted – its direction changes according to Snell’s law.
By measuring the angles and travel times of the refracted waves at many stations, scientists can map the boundaries where the speed changes, revealing the depth of the crust–mantle and mantle–core interfaces.
This pattern of refraction shows that the Earth is not homogeneous but consists of distinct layers – the crust, mantle and core – each with different physical properties.
When a wave passes from one layer into another with a different speed it is refracted – its direction changes according to Snell’s law.
By measuring the angles and travel times of the refracted waves at many stations, scientists can map the boundaries where the speed changes, revealing the depth of the crust–mantle and mantle–core interfaces.
This pattern of refraction shows that the Earth is not homogeneous but consists of distinct layers – the crust, mantle and core – each with different physical properties.
Examiner tips
- Use the phrase "different speeds" and "different densities" to show understanding of wave speed. Mention Snell’s law or the change in direction. Explain how travel‑time curves give boundary depths. Finish by stating the layered structure of the Earth.
Common mistakes
- Confusing refraction with reflection; students may say waves simply bounce. Failing to link speed change to density differences. Giving a general statement about layers without mentioning the evidence from refraction.
Mark scheme (4 marks)
- Seismic waves change speed / travel at different speeds in different layers of the Earth
- When seismic waves change speed they change direction / are refracted
- The pattern / path of refraction indicates where boundaries between layers are located
- This shows that the Earth is made up of distinct layers / has a layered structure (e.g. crust, mantle, core) with different properties / densities
Key terms in this question
seismic waves · refraction · internal structure
Related
- All WJEC GCSE Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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