Measuring Waves

This lesson covers the core practical and experimental methods for measuring wave properties — including using a ripple tank, measuring the speed of sound, and measuring wave speed on a string — as required by the Edexcel GCSE Physics specification (1PH0), Topic 4: Waves. You need to be able to describe these methods, calculate wave speed from measurements, and evaluate sources of error.

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Core Practical: Investigating Waves Using a Ripple Tank

A ripple tank is a shallow tray of water with a lamp above it and a white screen below. A vibrating dipper creates waves on the water surface, and the lamp projects the wave pattern onto the screen so it can be observed and measured.

Equipment

• Ripple tank (shallow tray of water)
• Motor with an attached dipper (to generate waves)
• Power supply with variable frequency control
• Lamp positioned above the tank
• White screen or paper below the tank
• Ruler (for measuring wavelength)
• Stopwatch (for measuring frequency)
• Stroboscope (optional — to "freeze" the wave pattern)

Method

1. Set up the ripple tank on a level surface and fill with water to a depth of approximately 5 mm.
2. Attach the dipper to the motor and switch on. Adjust the motor speed so that clear, regular waves are produced.
3. Position the lamp above the tank so that the wave pattern is projected onto the white screen below.
4. To measure wavelength: Use a ruler to measure the distance across several wave crests on the screen. Divide the total distance by the number of wavelengths measured to get the average wavelength. If the image is magnified, you must account for the magnification factor.
5. To measure frequency: Use a stopwatch to time how long it takes for a set number of waves (e.g. 10 waves) to pass a fixed point. Then calculate frequency using f = number of waves / time.
6. To calculate wave speed: Use the wave equation: v = f × λ.

Using a Stroboscope

A stroboscope is a flashing light. When its frequency matches the frequency of the waves, the waves appear to stand still ("freeze"). This makes it easier to measure the wavelength accurately.

Sources of Error and Improvements

Source of Error	Improvement
Waves reflect off the sides of the tank, causing interference	Place sloping barriers (sponges) at the edges to absorb reflections
Difficult to measure wavelength accurately with a ruler	Measure across multiple wavelengths and calculate the average
Parallax error when reading the ruler	View the ruler from directly above (perpendicular)
Motor vibrations cause the whole tank to shake	Secure the tank firmly to the bench
Wave pattern on screen may be magnified	Measure magnification factor and adjust calculation
Water depth not uniform	Ensure the tank is level before starting

Exam Tip: In the exam, you will be expected to describe this core practical and evaluate the method. Always mention measuring multiple wavelengths and dividing to get an average — this reduces the uncertainty in your measurement. Also mention using sponges to reduce reflections.

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Measuring the Speed of Sound in Air

There are two main methods for measuring the speed of sound in air at GCSE level.

Method 1: Timing Echoes

1. Stand a known distance (d) from a large flat wall (e.g. 50 m).
2. Clap or bang two blocks of wood together and listen for the echo.
3. Measure the time (t) for the sound to travel to the wall and back using a stopwatch.
4. The total distance travelled is 2d (to the wall and back).
5. Calculate speed: v = 2d / t.
6. Repeat multiple times and calculate a mean time to improve accuracy.

Method 2: Two Microphones and a Data Logger

1. Set up two microphones a measured distance (d) apart, connected to a computer or data logger.
2. Make a sharp sound (e.g. clap) at one end.
3. The data logger records the time difference (t) between the sound reaching the first and second microphone.
4. Calculate speed: v = d / t.

Sources of Error — Echo Method

Source of Error	Improvement
Human reaction time with the stopwatch (the largest source of error)	Repeat multiple times and calculate the mean; time many echoes and divide
Wind can affect the speed of sound	Conduct experiment on a calm day or indoors
Difficulty hearing the exact moment of the echo	Use two wooden blocks for a sharper sound

Sources of Error — Microphone Method

Source of Error	Improvement
Background noise may trigger microphones	Conduct in a quiet environment
Measurement of distance between microphones	Use a long ruler or tape measure carefully

Exam Tip: The microphone and data logger method is more accurate than the echo method because it removes human reaction time. When comparing methods, always state this as the key advantage.

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Measuring the Speed of Waves on a String

Method

1. Set up a length of string (or elastic cord) stretched between two fixed points. Measure the length of the string.
2. Pluck the string or send a single pulse along it.
3. Measure the time taken for the pulse to travel the length of the string and back.
4. Calculate the speed using: v = 2L / t (where L is the length of the string and t is the time for the pulse to travel there and back).
5. Alternatively, create a standing wave pattern using a vibration generator at a known frequency, measure the wavelength from the pattern, and use v = f × λ.

Using a Vibration Generator

1. Attach a vibration generator to one end of the string, with the other end passing over a pulley with a hanging mass.
2. Adjust the frequency of the vibration generator until a clear standing wave pattern is seen.
3. Measure the wavelength: the distance between adjacent nodes is half a wavelength, so measure the distance between two nodes and multiply by 2 to get λ.
4. Calculate wave speed: v = f × λ.

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Worked Example: Ripple Tank

A student observes waves in a ripple tank. She measures the distance across 6 complete wavelengths as 18 cm. She counts 20 waves passing a point in 10 seconds.

Step 1: Calculate wavelength.