Measuring wave speed: the core practical for measuring the speed, frequency and wavelength of waves in a solid and a fluid, and methods for the speed of sound and water ripples.

What this dot point is asking

Edexcel statement 4.7 and the associated core practical want you to describe how to measure the velocity of sound in air and of ripples on a water surface, and to investigate the suitability of equipment to measure the speed, frequency and wavelength of a wave in a solid and a fluid.

Measuring the speed of sound in air

The oscilloscope-and-microphones method is accurate because the oscilloscope can measure the very short time delay precisely. An alternative is the echo method: stand a known distance from a large flat wall, make a sound, and time how long the echo takes to return; the sound travels there and back, so the distance is twice the wall distance. Both methods give a value close to $330\,\text{m/s}$ for sound in air.

Measuring the speed of ripples on water

Water ripples move quickly and the pattern blurs, so a strobe light or a photograph is used to "freeze" the waves so the wavelength can be measured. Measuring the distance across, say, five wavelengths and dividing by five gives a more accurate wavelength than measuring a single wave, because the measurement error is shared over several wavelengths.

Waves in a solid

Across all three media (air, water and a solid string), the strategy is the same: measure frequency and wavelength (or distance and time) accurately, then apply a wave-speed equation. Part of the practical is judging which equipment is suitable, for example whether a ruler, a strobe or an oscilloscope gives the best measurement for that wave.

How Edexcel examines this

This core practical is examined on both tiers as a method question worth four to six marks, asking you to describe how to measure a wave speed and to state the measurements and equation used. Mark schemes reward a clear, ordered method: the quantities measured (a distance and a time, or a frequency and a wavelength), the instrument used, the equation ($v = \frac{x}{t}$ or $v = f\lambda$), and a step that improves accuracy or reliability such as measuring across several wavelengths or repeating and averaging. For the speed of sound, both the two-microphone oscilloscope method and the echo method are creditworthy, but the echo method must double the wall distance. For ripples, examiners reward using a strobe or photograph to freeze the pattern and measuring several wavelengths. You may also be asked to identify the main sources of error (reaction-time error in manual timing, blurred ripple patterns) and how to reduce them, which is why the precise oscilloscope timing and multi-wavelength measurements are preferred.

Try this

Q1. State the equation used to find a wave speed from its frequency and wavelength. [1 mark]

• Cue. $v = f \times \lambda$.

Q2. Explain why you should measure across several wavelengths rather than one in a ripple tank. [1 mark]

• Cue. It reduces the percentage (measurement) error in the wavelength.