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3.1.2Waves

Wave Motion — Oxford AQA International GCSE Combined Science - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 3 of 13

📚 Oxford AQA International GCSE Combined Science - Physics Hub / Topical Revision
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Section 1 of 9The key quantities

Several terms describe any regular wave. Make sure you can define each and give its unit.

Amplitude (A)
Amplitude (A) the maximum displacement of a point on the wave from its rest (equilibrium) position. Unit: metres for a water wave. A larger amplitude means more energy.
Wavelength (λ)
Wavelength (λ) the distance between two identical points on adjacent waves, for example crest to crest or compression to compression. Unit: metres (m).
Frequency (f)
Frequency (f) the number of complete waves passing a point each second, or produced by the source each second. Unit: hertz (Hz), where 1 Hz = 1 wave per second.
Period (T)
Period (T) the time for one complete wave to pass a point. Unit: seconds (s).
Wave speed (v)
Wave speed (v) the distance travelled by a wave crest each second. Unit: m s−1.

Section 2 of 9Frequency and period

Frequency and period are reciprocals of each other:

f = 1 / T and T = 1 / f

If 50 waves pass each second, the frequency is 50 Hz and the period is 1/50 = 0.020 s. Higher frequencies are written using kilohertz (kHz, 103 Hz), megahertz (MHz, 106 Hz) and gigahertz (GHz, 109 Hz).

Section 3 of 9Reading a wave graph

A displacement–distance graph is like a freeze-frame photograph of the wave at one instant.

The wavelength is read as the horizontal distance between successive peaks, and the amplitude is the height from the centre line to a peak.
Time
A displacement–time graph shows the motion of one point over time.
Period
On this graph the horizontal distance between successive peaks is the period, not the wavelength.
This distinction is a common source of marks lost in examinations: always check what is on the horizontal axis.

Amplitude is half of the peak-to-peak height. If the peaks are 6 cm above and 6 cm below the centre line, the amplitude is 6 cm, not 12 cm.

Section 4 of 9Wavefronts and phase

Wavefronts are lines drawn on a diagram joining points at the same stage of the wave, such as the crests of ripples seen from above.

1
The distance between adjacent wavefronts is the wavelength.
2In phase
Two points on a wave that are at the same stage of the cycle, such as two crests, are in phase.
3
Points that are half a wavelength apart, like a crest and a trough, are in antiphase.

Water waves are investigated in a ripple tank, where a motor-driven bar produces plane waves of a chosen frequency. A lamp above the tank projects the wavefronts onto a white sheet below. A stroboscope or a video can be used to "freeze" the pattern so that the wavelength can be measured accurately.

Section 5 of 9Putting the quantities together

The quantities are not independent.

1
Wave speed, frequency and wavelength are linked by an equation, covered in the next lesson (The Wave Equation).
2
A wave of higher frequency has a shorter period and, for the same speed in the same medium, a shorter wavelength.
3
The energy carried depends on the amplitude.
4
For light, a brighter beam means a larger amplitude; for sound, a louder note means a larger amplitude (see Pitch and Loudness).

Section 6 of 9Worked ideas: reading an oscilloscope or a ripple photograph

Wave quantities appear in many practical settings.

1
In a ripple tank photograph, the distance between several wavefronts divided by the number of gaps gives a more accurate wavelength than a single gap.
2
If 6 gaps measure 9.0 cm, one wavelength is 1.5 cm.
3
For a sound wave displayed on an oscilloscope, the horizontal axis is time, so the distance between peaks multiplied by the time-base setting gives the period, and the reciprocal gives the frequency.
4
A loud sound gives a taller trace (larger amplitude) and a high-pitched sound gives a trace with more peaks across the screen (higher frequency).

When describing waves, always give the quantity and its unit: wavelength in m, frequency in Hz, period in s, amplitude in m or the unit of whatever is oscillating.

Worked ideas: reading an oscilloscope or a ripple photograph
Be careful with sub-multiples, since a wavelength of 0.5 cm is 0.005 m, and a frequency of 3 kHz is 3000 Hz.
A quick check on the size of an answer helps: sound wavelengths are of the order of metres, light wavelengths of the order of hundreds of nanometres.

Section 7 of 9Worked examples

  1. Five complete waves pass a point in 2.0 s. Find the frequency and period.

  2. A displacement–distance graph shows crests at 0.0 m, 0.80 m and 1.60 m, with the crest height 3.0 cm and trough depth 3.0 cm. State the wavelength and amplitude.

  3. A source vibrates at 25 kHz. Calculate its period.

Section 8 of 9Key terms

TermMeaning
AmplitudeMaximum displacement from the rest position
WavelengthDistance between two successive identical points on a wave
FrequencyNumber of waves per second; hertz
PeriodTime for one complete wave
WavefrontLine joining points in phase on a wave
PhaseStage reached in the cycle by a point on a wave

Section 9 of 9Exam technique

1
Measuring the amplitude from trough to crest – it is from the centre line to the crest.
2
Reading the wavelength from a displacement–time graph.
3
Mixing up frequency and period.
4
Forgetting to convert kHz or MHz to Hz.
5
Saying that a larger amplitude changes the frequency or speed – it only changes the energy.

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