Explain why the speed of a wave changes when it passes from one medium into another, and state what happens to the wavelength and frequency during this transition.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A plane wave travelling through air enters a denser medium such as water at normal incidence.
Model answer (4 marks)
The speed of a wave depends on the properties of the medium through which it travels, such as density and elasticity (or bulk modulus). When a plane wave enters a denser medium its speed decreases – for sound this is because the higher density and bulk modulus reduce the speed, and for light the higher optical density reduces the speed. The frequency of the wave is set by the source and therefore remains unchanged when the medium changes. Since v = fλ and f is constant, the wavelength must decrease in proportion to the decrease in speed.
Examiner tips
- Use the formula v = fλ to link speed, frequency and wavelength.
- Mention the medium’s density or bulk modulus as the cause of the speed change.
- State that frequency is unchanged because it is fixed by the source.
- Show the wavelength decreases because v decreases while f stays constant.
Common mistakes
- Saying the frequency changes – it does not.
- Confusing the speed of light with the speed of sound – both change but for different reasons.
- Ignoring the role of medium properties such as density or bulk modulus.
Mark scheme (4 marks)
- The speed of a wave depends on the properties of the medium through which it travels (e.g. density, elasticity/bulk modulus).
- As the wave enters the denser medium its speed decreases (water is denser / has a higher bulk modulus effect reducing speed for sound, or greater optical density reducing speed for light).
- The frequency remains unchanged because it is determined by the source of the wave, not the medium.
- Since v = fλ and frequency is constant, the wavelength decreases in proportion to the decrease in speed.
Key terms in this question
frequency · wavelength · medium
Related
- All IB DP Physics Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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