A student plucks a string on a guitar, producing a wave that travels along the string. Describe what is meant by the amplitude, wavelength and frequency of a wave, and explain how increasing the frequency of a wave affects its wavelength when the wave speed stays the same.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Amplitude is the maximum displacement of a point on the wave from its rest/equilibrium position.
Wavelength is the distance between two successive points that are in phase – the length of one complete wave cycle.
Frequency is the number of complete waves (cycles) passing a point per second, or produced per second.
Wave speed is given by v = f λ. If the speed v remains constant and the frequency f increases, the wavelength λ must decrease.
Thus, when the frequency rises while the wave speed stays the same, the wavelength becomes shorter because the two quantities are inversely proportional.
Wavelength is the distance between two successive points that are in phase – the length of one complete wave cycle.
Frequency is the number of complete waves (cycles) passing a point per second, or produced per second.
Wave speed is given by v = f λ. If the speed v remains constant and the frequency f increases, the wavelength λ must decrease.
Thus, when the frequency rises while the wave speed stays the same, the wavelength becomes shorter because the two quantities are inversely proportional.
Examiner tips
- Use the exact definitions for amplitude, wavelength and frequency. Show the wave‑speed equation and state the relationship between f and λ. Explain qualitatively that λ decreases when f increases at constant v. Keep the answer concise – 5 marks can be earned with 5 clear points.
Common mistakes
- Confusing amplitude with wavelength or frequency. Using the wrong wave‑speed formula (e.g., v = λ/f). Failing to mention that the wavelength decreases when frequency increases at constant speed.
Mark scheme (5 marks)
- Amplitude is the maximum displacement of a point on the wave from its rest/equilibrium position
- Wavelength is the distance between two successive points that are in phase / the length of one complete wave cycle
- Frequency is the number of complete waves (cycles) passing a point per second / produced per second
- Correct use of wave speed equation: wave speed = frequency × wavelength (v = f λ), stating that if speed is constant and frequency increases, wavelength must decrease
- Clear qualitative explanation that wavelength decreases because the two quantities are inversely proportional (at constant wave speed)
Key terms in this question
amplitude · wavelength · frequency · wave speed
Related
- All Eduqas A-Level Physics revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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