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Edexcel IGCSE Physics revision

Edexcel IGCSE Physics Waves units questions

Revise the syllabus content for Waves units, learn how to approach common exam questions, and study two worked examples with clear diagrams.

Edexcel IGCSE Physics Subtopic 3.a

Waves units syllabus

Questions on waves units can test recall, calculations, explanations, diagrams, data handling and practical skills. You should be able to:

  • 3.1 use the following units: degree (°), hertz (Hz), metre (m), metre/second (m/s) and second (s)

How to answer waves units questions

  1. Write the unit symbol exactly and distinguish quantities that use similar-looking units.
  2. Convert prefixes and compound units before substituting values into an equation.
  3. Check that the unit of the final answer matches the quantity asked for.
  4. Use the context of a diagram or calculation to reject units with the wrong dimensions.

Waves units example questions and worked answers

These examples show how information in a diagram, graph or experimental context becomes part of a complete exam answer.

Example 1: Using a Diagram

Question 1

A student uses this method to investigate the speed of sound in air.
  • set up an oscilloscope to detect and display a sound wave
  • use a computer and a speaker to produce a sound of known wavelength
  • use the oscilloscope to determine the frequency of the sound wave
  • use a formula to calculate the speed of sound
The diagram shows the oscilloscope screen and the oscilloscope settings.
Oscilloscope screen showing a sinusoidal wave across a gridded display. Settings shown: y direction: 1 square = 5 mV; x direction: 1 square = 0.5 ms. Approximately five complete wave cycles are visible across the screen width, with roughly two horizontal squares per cycle.
The wavelength of the sound wave is 27cm.
Calculate the speed of sound.
speed of sound = m/s

Final answer

Speed of sound = 340 m/s

Using the unrounded value gives 337.5 m/s, which is also accepted.

Mark scheme points

  1. M1 Use the relationship speed = frequency × wavelength.
  2. M2 Substitute the frequency and wavelength correctly: 1250 × 0.27.
  3. M3 Obtain a speed of 340 m/s or 337.5 m/s.

Explanation

The frequency found from the oscilloscope trace is 1250 Hz. Convert the wavelength from centimetres to metres before substituting:

wavelength = 27 cm = 0.27 m
speed = frequency × wavelength
speed = 1250 × 0.27
speed = 337.5 m/s ≈ 340 m/s

The answer must be given in metres per second because the wavelength was converted to metres.

Common mistakes

  • Using 27 instead of 0.27 for the wavelength, causing a factor-of-100 error.
  • Using the period instead of the frequency in the speed equation.
  • Writing the formula without substituting the numerical values or omitting the unit m/s.
  • Examiners reported that candidates who had calculated a frequency generally used it correctly in this part, so show the frequency clearly in your working.

Example 2: Using a Diagram

Question 2

A student uses an oscilloscope to determine the speed of sound.

The diagram shows the oscilloscope trace produced by the sound wave.

Oscilloscope trace of a sinusoidal sound wave on a square grid. The horizontal distance between two successive peaks is 4 squares. Oscilloscope x direction: 1 square = 1 ms; y direction: 1 square = 1 mV.

Oscilloscope settings

y direction: 1 square = 1mV

x direction: 1 square = 1ms

The student uses two microphones and a ruler to determine the wavelength of the sound wave.

He finds that the wavelength is 1.4m.

Use the oscilloscope trace to calculate the speed of the wave.

speed = m/s

Final answer

Period = 4 × 1 ms = 4 ms = 0.004 s
frequency = 1 ÷ 0.004 = 250 Hz
speed = frequency × wavelength
       = 250 × 1.4
       = 350 m/s

speed = 350 m/s

Mark scheme points

  1. M1 Determine the period as 4 squares.
  2. M2 Convert the period: 4 ms = 0.004 s.
  3. M3 Calculate the frequency using f = 1/T = 250 Hz.
  4. M4 Substitute into v = f × λ, using λ = 1.4 m.
  5. M5 Evaluate the speed as 350 m/s.

Explanation

One complete cycle is the horizontal distance from one peak to the next peak. This is 4 squares. Since each square represents 1 ms, the period is 4 ms, which must be changed to 0.004 s before finding the frequency.

Use the reciprocal relationship between period and frequency, then use the wave equation with the given wavelength:

T = 0.004 s
f = 1/T = 250 Hz
v = fλ = 250 × 1.4 = 350 m/s
  • Count the squares for one complete cycle.
  • Convert milliseconds to seconds.
  • Find the frequency before calculating the speed.
  • Give the final answer in m/s.

Common mistakes

  • Using the distance from a peak to the neighbouring trough instead of one complete peak-to-peak cycle.
  • Forgetting to convert 4 ms into 0.004 s before calculating the frequency.
  • Using f = T instead of f = 1/T.
  • Using the oscilloscope’s vertical scale; only the horizontal scale is needed to find the period.
  • Examiners reported that few students stated how many squares represented one period, and calculating the correct frequency was the most common difficulty. Always show the 4-square period clearly.

Practise Waves units questions

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