Waves in a ripple tank and a solid
GCSE Physics (8463) · Required practical 8 — method, variables, the marks examiners report students losing.
Measure the frequency, wavelength and speed of waves in a ripple tank (water waves) and in a stretched string, using wave speed = frequency x wavelength.
Apparatus
- Ripple tank with a lamp above, a motor and dipper, and a power supply
- White screen or paper below the tank, and a ruler
- Stroboscope (or a phone camera) to freeze the wave pattern
- For the string: signal generator, vibration generator, string, pulley and masses
Method
- 1Ripple tank: switch on the dipper to make waves and shine the lamp through so wavefronts show on the screen below.
- 2Measure the wavelength by measuring the distance across several wavefronts and dividing; read the frequency from the motor setting (or use a strobe to freeze the pattern).
- 3Calculate wave speed = frequency x wavelength.
- 4Waves in a solid: attach the string to a vibration generator, run it over a pulley with masses, and adjust the signal generator until a clear standing wave forms.
- 5The wavelength is twice the distance between adjacent nodes; take the frequency from the generator and again use wave speed = frequency x wavelength.
Results & processing
- Wave speed = frequency x wavelength (v = f x wavelength).
- Measure across several wavelengths and divide to reduce the percentage error; on a string, wavelength = 2 x the distance between adjacent nodes.
Where students lose marks
Measuring only one wavelength.
Fix: Measure across several wavelengths and divide by the number, which reduces the percentage (measurement) error.
Confusing the frequency of the source with the wavelength.
Fix: Frequency comes from the motor or signal generator; wavelength is a distance you measure on the pattern.
On a string, taking node-to-node distance as the whole wavelength.
Fix: The distance between adjacent nodes is half a wavelength, so wavelength = 2 x that distance.
Improve the method
- Measure across as many wavelengths as possible, then divide.
- Use a stroboscope to freeze the ripple pattern so it can be measured.
- Repeat measurements and take a mean; use a shadow/screen so the wavefronts are clear.
Try it — exam-style
Water waves in a ripple tank have a wavelength of 0.02 m and a frequency of 15 Hz. Calculate the wave speed.
A student measures the distance across 10 wavelengths as 0.60 m. State the wavelength, and explain why measuring across 10 wavelengths is better than measuring one.
Questions are written in the style of past AQA papers — never copied from them.
Drill it properly
Stuck on waves in a ripple tank and a solid?
Wave-measurement questions reward the many-wavelengths trick and the node-to-node rule — I drill both, and your first lesson is free.