A sound wave travels through air. The sound wave has a wavelength of 0.85 m and a wave speed of 340 m/s. Explain what is meant by the terms 'wavelength', 'frequency', and 'wave speed', and state the relationship between these three quantities.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Sound waves are longitudinal waves that transfer energy through the air without transferring matter.
Model answer (5 marks)
Wavelength is the distance between two successive points that are in phase, for example from one compression to the next compression in a sound wave.
Frequency is the number of complete cycles that pass a fixed point each second.
Wave speed is the distance that the wave front travels through the medium per second.
The three quantities are related by the equation v = f × λ.
Thus, if the wave speed is constant, an increase in frequency must be accompanied by a decrease in wavelength, and vice‑versa.
Frequency is the number of complete cycles that pass a fixed point each second.
Wave speed is the distance that the wave front travels through the medium per second.
The three quantities are related by the equation v = f × λ.
Thus, if the wave speed is constant, an increase in frequency must be accompanied by a decrease in wavelength, and vice‑versa.
Examiner tips
- Define each term clearly and use the correct units (m, s⁻¹, Hz).
- State the formula v = f × λ and explain the inverse relationship when v is constant.
- Use the example values to show the relationship if time permits.
- Avoid unnecessary wording – keep it concise and to the point.
Common mistakes
- Confusing wavelength with wavelength of a different wave type or giving the wrong definition (e.g. peak to trough).
- Using the wrong formula (v = λ / f) or omitting the units.
- Failing to mention the inverse relationship between frequency and wavelength at constant speed.
Mark scheme (5 marks)
- Wavelength is the distance from one point on a wave to the next identical point (e.g. crest to crest or compression to compression)
- Frequency is the number of complete waves (cycles) passing a point per second
- Wave speed is the distance travelled by the wave per second (or the speed at which the wave front moves through the medium)
- Wave speed = frequency × wavelength (v = f × λ)
- Correct application of the relationship: as frequency increases, wavelength decreases (for a constant wave speed) OR correct identification that frequency and wavelength are inversely proportional at constant speed
Key terms in this question
wavelength · frequency · wave speed
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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