AQA · A-Level · Physics
Course version: 7408 · Version from Sept 2016
Oscillation of the particles of the medium; amplitude, frequency,…
Full specification wording
Progressive waves: Oscillation of the particles of the medium; amplitude, frequency, wavelength, speed, phase, phase difference, c = f λ f = T Phase difference may be measured as angles (radians and degrees) or as fractions of a cycle.
A progressive wave transfers energy through a medium as particles oscillate about fixed equilibrium positions; the wave pattern travels while the particles themselves do not travel with it.
Checked against the AQA 7408 specification (opens in a new tab) on .
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A progressive wave transfers energy through a medium as particles oscillate about fixed equilibrium positions; the wave pattern travels while the particles themselves do not travel with it. Amplitude is the maximum displacement from equilibrium, frequency f is oscillations per second, wavelength λ is the distance between successive points in phase, and speed c is how fast a crest advances. Phase describes where a particle sits within its cycle; phase difference compares two points, measured in radians, degrees or fractions of a cycle. The wave equation c = f λ links speed, frequency and wavelength, and f = 1/T links frequency to period.
For example: A wave of frequency 50 Hz and wavelength 0.40 m travels at c = 50 × 0.40 = 20 m s⁻¹.
What earns the marks
- States that particles oscillate about equilibrium positions and that energy, not the particles, is transferred along the wave.
- Defines amplitude as maximum displacement from equilibrium and wavelength as distance between successive points in phase.
- Defines frequency as oscillations per second and period as the time for one complete oscillation, using f = 1/T.
- Applies c = f λ correctly, including unit consistency, to find speed, frequency or wavelength.
- Describes phase difference as a fraction of a cycle and converts between fractions, degrees and radians, e.g. one quarter cycle = 90° = π/2 rad.
Common mistakes and how to put them right
Treating particles as travelling with the wave.
Put it right: Particles oscillate about fixed equilibrium positions while the wave pattern and energy advance.
Confusing period with frequency.
Put it right: Period is the time for one cycle and frequency is the reciprocal of period.
Measuring wavelength between a crest and the next trough.
Put it right: Measure between successive crests, successive troughs or any two successive points in phase.
Exam tips
- Write the wave equation as c = f λ and substitute values with units before calculating.
- When asked for phase difference, state the unit clearly, choosing radians, degrees or a fraction of a cycle as requested.
- Sketch or annotate a displacement–distance graph to show amplitude and wavelength before using them in calculations.
Try the lesson
The start of the Progressive waves: Oscillation of the particles of the medium; amplitude, frequency, wavelength, speed, phase, phase difference, c = f λ f = T Phase difference may be measured as angles (radians and degrees) or as fractions of a cycle. lesson, made from checked teaching notes for this specification point.
Have a go first
A wave travels along a rope. What is meant by the amplitude of the oscillation of a particle of the rope?
Listen
A progressive wave transfers energy through a medium as particles oscillate about fixed equilibrium positions; the wave pattern travels while the particles themselves do not travel with it. Amplitude is the maximum displacement from equilibrium, frequency f is oscillations per second, wavelength λ is the distance between successive points in phase, and speed c is how fast a crest advances. Phase describes where a particle sits within its cycle; phase difference compares two points, measured in radians, degrees or fractions of a cycle. The wave equation c = f λ links speed, frequency and wavelength, and f = 1/T links frequency to period. For example, a wave of frequency 50 Hz and wavelength 0.40 m travels at c = 50 × 0.40 = 20 m s⁻¹. A strong answer earns marks for these points. States that particles oscillate about equilibrium positions and that energy, not the particles, is transferred along the wave. Defines amplitude as maximum displacement from equilibrium and wavelength as distance between successive points in phase. Defines frequency as oscillations per second and period as the time for one complete oscillation, using f = 1/T. Applies c = f λ correctly, including unit consistency, to find speed, frequency or wavelength. A common slip: Treating particles as travelling with the wave. Instead, particles oscillate about fixed equilibrium positions while the wave pattern and energy advance. Exam tip: Write the wave equation as c = f λ and substitute values with units before calculating.
Key words
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Frequency
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Wavelength
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Phase difference
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See every definition
- Frequency
- The number of oscillations per second, linked to period by f = 1/T.
- Wavelength
- The distance between successive points in phase.
- Phase difference
- A comparison of two points measured in radians, degrees or fractions of a cycle.
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