Explain how the wavelength and frequency of a wave are related to its speed, and describe what happens to the wavelength of a wave when it passes from a medium in which it travels slowly into a medium in which it travels faster, given that the frequency remains constant.

IB DP Physics Standard Level (2023 syllabus) — C.2 Wave model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

Wave speed equals frequency multiplied by wavelength (v = fλ). Frequency is set by the source and remains unchanged when the wave enters a new medium. If the wave speed increases while the frequency stays constant, the wavelength must increase. Thus, wavelength is directly proportional to speed at constant frequency, so it increases by the same factor as the speed.

Examiner tips

  • State v = fλ first. Mention frequency is source‑determined. Explain that speed change forces λ to change. Show proportionality λ ∝ v at constant f. Keep answer concise to hit all marks.

Common mistakes

  • Confusing frequency with wavelength change. Writing v = λf without noting the order. Forgetting to state that frequency remains constant.

Mark scheme (4 marks)

  1. Wave speed equals frequency multiplied by wavelength (v = fλ)
  2. Frequency is determined by the source and does not change when the wave moves into a new medium
  3. When wave speed increases and frequency is constant, wavelength must increase
  4. Explicit proportional reasoning: wavelength is directly proportional to speed (at constant frequency), so wavelength increases by the same factor as the speed increases

Key terms in this question

frequency · wavelength · medium

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