Required practical activity 20: make observations to identify the…

The idea

This practical develops your ability to judge whether apparatus is fit for purpose and to obtain reliable measurements of wave properties. In a ripple tank, a vibrating bar or dipper creates water waves; you observe the pattern and decide whether a stroboscope, a slow-motion camera or a metre rule and timer are suitable for measuring wavelength and frequency. For waves in a solid, a vibration generator attached to a string produces a stationary wave; you assess whether a signal generator, pulley and ruler allow you to measure wavelength and frequency accurately. You then calculate speed using v = f λ. Suitability depends on resolution, range, stability and whether the wave pattern is clear enough to measure. You must record raw measurements with units and consider repeats to reduce random error.

What earns the marks

• Identifies that wavelength in a ripple tank can be measured by recording the distance across several wavefronts and dividing by the number of intervals, improving precision compared with measuring one wavelength.
• Explains that frequency in a ripple tank may be determined from the driving frequency of the vibrator or by timing multiple oscillations and dividing by the number of cycles.
• Assesses apparatus suitability by considering resolution, range, stability and whether the wave pattern is sufficiently clear to locate crests or nodes.
• Describes how a vibration generator and signal generator produce stationary waves on a string, and how nodes and antinodes allow wavelength to be measured.
• Calculates wave speed using v = f λ after obtaining consistent values for frequency and wavelength, with correct units.
• Evaluates sources of uncertainty such as parallax, blurred images, slipping string, water ripples reflecting, and suggests repeats or improved techniques.

Common mistakes and how to put them right

• Measuring a single wavelength in a ripple tank and treating it as precise.

  Put it right: Measure across several wavefronts and divide by the number of intervals to reduce the effect of reading error.

• Confusing frequency with the number of waves seen.

  Put it right: Frequency is the number of complete oscillations per second, so time several oscillations and divide by the number of cycles.

• Using v = f λ with wavelength in centimetres and frequency in hertz without converting.

  Put it right: Convert wavelength to metres before calculating speed in m/s.

• Assuming any apparatus is suitable without checking its range or resolution.

  Put it right: Justify suitability by comparing the apparatus scale and response to the size and rate of the wave being measured.

Exam tips

• When asked to identify suitable apparatus, name the instrument and give a reason linked to resolution, range or stability.
• Show the calculation v = f λ with substituted values and units; keep wavelength in metres and frequency in hertz.
• Describe how you would reduce random error, for example by repeating measurements and calculating a mean, or by measuring across multiple waves.