A student observes that when a wave undergoes reflection at a smooth, plane surface, three wave properties remain unchanged. State these three properties and explain why the frequency cannot change during reflection.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
1. Speed
2. Wavelength
3. Frequency
The frequency cannot change because the wave remains in the same medium after reflection. Since the speed of the wave in that medium is unchanged and the wavelength is unchanged, the relationship (v = flambda) requires the frequency to remain constant. The source of the wave determines the frequency, and reflection does not alter the source or the medium, so no change in frequency occurs.
2. Wavelength
3. Frequency
The frequency cannot change because the wave remains in the same medium after reflection. Since the speed of the wave in that medium is unchanged and the wavelength is unchanged, the relationship (v = flambda) requires the frequency to remain constant. The source of the wave determines the frequency, and reflection does not alter the source or the medium, so no change in frequency occurs.
Examiner tips
- Use the exact wording: speed, wavelength, frequency. Explain that the wave stays in the same medium, so speed is unchanged. Show the equation (v = flambda) to link speed, wavelength and frequency. Mention that the source sets the frequency, not the reflection.
- common_mistakes
- :
- Saying the wave changes medium. Forgetting to state all three properties. Using the wrong relationship (e.g., saying frequency changes with speed).
Mark scheme (4 marks)
- States that speed is unchanged during reflection
- States that wavelength is unchanged during reflection
- States that frequency is unchanged during reflection
- Explains that the wave stays in the same medium after reflection, so its speed does not change, and since speed and wavelength are both unchanged, frequency must also remain constant (accept: the wave does not change medium so nothing drives a change in frequency / frequency is a property of the source not the medium)
Key terms in this question
Related
- All Cambridge International IGCSE Physics (0625) revision notes →
- How to answer a "State and Explain" question →
- Decode the mark scheme abbreviations →
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