Required Practical: Waves in a Ripple Tank and on a String

This lesson covers the AQA GCSE Combined Science Trilogy (8464) Required Practical on measuring the speed, frequency and wavelength of waves in a ripple tank and waves on a vibrating string. This is one of the required practicals you must know for the exam — you may be asked to describe the method, identify variables, explain how to improve accuracy, or analyse data.

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Overview

AQA requires you to investigate waves using two set-ups:

1. Ripple tank — measuring the speed and wavelength of water waves.
2. Vibrating string — measuring the frequency and wavelength of standing waves on a string.

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Part 1 — Waves in a Ripple Tank

Equipment

Item	Purpose
Ripple tank (shallow tray of water)	Contains the water through which waves travel
Motor and vibrating dipper (straight bar)	Generates regular, parallel water waves
Power supply and rheostat / variable resistor	Controls the frequency of the dipper
Lamp (placed above the tank)	Projects wave pattern shadows onto paper below the tank
White paper / screen (below the tank)	Displays the wave pattern for measurement
Ruler	Measures distances (for wavelength)
Stroboscope (optional)	Flashes light at the wave frequency to make waves appear stationary
Stopwatch	Measures time for wave fronts to travel a known distance

Method

1. Set up the ripple tank with a uniform, shallow depth of water (about 5 mm).
2. Switch on the motor so the dipper produces regular, straight wave fronts.
3. Place the lamp above the tank so that wave shadows are projected onto the paper below.
4. Measuring wavelength: Use a ruler to measure the distance covered by several wavelengths (e.g. 10 wave shadows). Divide the total distance by the number of wavelengths to get the wavelength.
5. Measuring wave speed: Mark two lines a known distance apart on the paper. Time how long it takes for a wavefront to travel between the two lines. Calculate speed = distance / time.
6. Alternatively, use $v = f\lambda$v=fλ if you know the frequency setting and have measured the wavelength.

Improving Accuracy

• Measure the distance for several wavelengths and divide — this reduces the percentage error compared to measuring a single wavelength.
• Use a stroboscope to "freeze" the wave pattern, making it easier to measure wavelength.
• Repeat measurements and calculate a mean.
• Ensure the water depth is uniform to avoid refraction effects.
• Dim the room lights so the projected wave pattern is clearly visible.

Variables

Variable type	Variable
Independent	Frequency of the dipper (changed by adjusting the power supply)
Dependent	Wavelength and wave speed
Control	Depth of water, temperature of water

Exam Tip (AQA 8464): In a 6-mark practical question, always mention: the key measurements, how you ensure accuracy (e.g. measuring multiple wavelengths), control variables, and how you would display and analyse the results.

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Part 2 — Waves on a Vibrating String

Equipment

Item	Purpose
Signal generator	Controls the frequency of vibration
Vibration generator (mechanical oscillator)	Vibrates one end of the string
String / elastic cord	The medium through which the wave travels
Pulley and masses	Keep the string under tension
Ruler / metre rule	Measures the length of the string and distance between nodes
Retort stand and clamp	Supports the pulley

Method

1. Attach a string to the vibration generator at one end and pass it over a pulley with a hanging mass at the other end.
2. Switch on the signal generator and set a frequency. Adjust the frequency until a clear standing wave pattern appears (the string vibrates with distinct nodes and antinodes).
3. The distance between two adjacent nodes is half a wavelength ($\lambda / 2$λ/2).
4. Measure the distance between several nodes using a ruler and calculate the wavelength.
5. Read the frequency from the signal generator.
6. Calculate the wave speed: $v = f\lambda$v=fλ.
7. Change the frequency and repeat to investigate how wavelength changes with frequency at a constant wave speed.

Key Definitions for Standing Waves

• Node — a point on the string that does not move (zero displacement).
• Antinode — a point of maximum displacement.
• The distance between two consecutive nodes = $\lambda / 2$λ/2.

Variables

Variable type	Variable
Independent	Frequency (set on the signal generator)
Dependent	Wavelength (calculated from node spacing)
Control	Length of string, tension (mass), type of string

Exam Tip: When the frequency is increased (at constant tension), the wavelength decreases but the wave speed stays approximately the same. This is because $v = f\lambda$v=fλ and the wave speed on a string depends on the tension and the mass per unit length, not the frequency.

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Analysing Results

You can plot a graph of wavelength (y-axis) against 1/frequency (x-axis). If $v = f\lambda$v=fλ, then $\lambda = v/f = v \times (1/f)$λ=v/f=v×(1/f). This means the graph should be a straight line through the origin, and the gradient equals the wave speed.

graph TD
    A["Measure wavelength λ and frequency f"] --> B["Calculate 1/f"]
    B --> C["Plot λ (y) vs 1/f (x)"]
    C --> D["Gradient = wave speed v"]

    style A fill:#2c3e50,color:#fff
    style B fill:#2980b9,color:#fff
    style C fill:#27ae60,color:#fff
    style D fill:#8e44ad,color:#fff

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Safety Precautions

Hazard	Precaution
Water spillage from ripple tank	Mop up spills immediately; keep electrical equipment away from water
Electrical equipment near water	Use a low-voltage power supply; do not touch with wet hands
Hanging masses falling	Place a cushion or sand tray below the masses
Vibrating string snapping	Wear eye protection; do not lean over the string

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Common Mistakes