Required practical 20 · Physics · Physics Paper 2, both tiers
Waves in a ripple tank and a solid
Make observations to find the frequency, wavelength and speed of waves in a ripple tank and waves in a solid.
The exam trap. The wave carries energy along; the water and the string only move up and down. Use metres in v = f λ.
Variables
- Independent (you change)
- None is changed on purpose: this practical measures frequency and wavelength to find the wave speed
- Dependent (you measure)
- Wavelength, and so wave speed
- Control (you keep the same)
- The same depth of water in the ripple tank
- The same tension and length of string
- Measuring across several waves
Apparatus
- Ripple tank with a motor-driven dipper and lamp
- Stroboscope or phone camera
- Metre rule
- Signal generator and vibration generator
- String or elastic cord, pulley and masses
Method, and why each step matters
Ripple tank: switch on the dipper and light above the tank so the waves cast shadows on paper below.
Why The shadows make the wavefronts easy to see and measure.
Measure the distance across 10 wavefronts and divide by 10.
Why Measuring many waves reduces the uncertainty in one small wavelength.
Count the waves passing a point in 10 seconds to find the frequency.
Why Frequency is waves per second.
Calculate wave speed = frequency × wavelength.
Why Wave speed is frequency times wavelength: v = f λ, with λ in metres for m/s.
String: set up the string over a pulley with masses, attached to a vibration generator, and adjust the frequency until a clear standing wave forms.
Why A steady pattern makes the wavelength measurable.
Measure the length of several half wavelengths, find λ and use the signal generator frequency to calculate v.
Why The frequency is read directly from the signal generator.
Risks
| Hazard | How to reduce the risk |
|---|---|
| Water near electrical equipment in the ripple tank. | Keep the power supply dry and away from the tank. |
| Masses on the string can fall. | Put a cushion underneath. |
A worked set of results
| Measurement | Ripple tank | String |
|---|---|---|
| Distance measured | 15 cm across 10 waves | 1.20 m for 3 half wavelengths |
| Wavelength | 1.5 cm | 0.80 m |
| Frequency | 8.0 Hz | 30 Hz |
| Wave speed | 0.12 m/s | 24 m/s |
- Water waves: 8.0 × 0.015 = 0.12 m/s.
- It is the wave (energy) that travels; the water and the string only oscillate about fixed positions.
Mistakes that cost marks
Converting wavelength wrongly: 1.5 cm is 0.015 m.
Instead Use metres in v = f λ to get m/s.
Reporting the full range as the uncertainty.
Instead Uncertainty is half the range of repeat readings.
Thinking the water moves along with the wave.
Instead The water moves up and down; the energy travels.