PAG: Measuring Wave Properties (OCR Gateway GCSE Physics: Combined Science)

Equipment List

• Resolution of measuring equipment:
  • Metre ruler = 1 mm
  • Stopwatch = 0.01 s

Aims of the Experiment

• To measure frequency, wavelength and wave speed by observing water waves in a ripple tank

Variables

• Independent variable = frequency, f
• Dependent variable = wavelength, λ
• Control variables:
  • Same depth of water
  • Same temperature of water

Method

Set up of ripple tank to investigate wave properties

1. Set up the apparatus as shown and fill the ripple tank with water to a depth of no more than 1 cm
2. Turn on the power supply and the light source to produce a wave pattern on the screen
3. The wavelength of the waves can be determined by using a ruler to measure the length of the screen and dividing this distance by the number of wavefronts
4. The frequency can be determined by timing how long it takes for a given number of waves to pass a particular point and dividing the number of wavefronts by the time taken
5. Record the frequency and wavelength in a table and repeat the measurements

• An example of the data collection table is shown below:

Analysis of Results

• The speed of the waves can be determined using the equation:

Wave Speed = Frequency × Wavelength

v = fλ

• Where:
  • v = wave speed in metres per second (m/s)
  • f = frequency in Hertz (Hz)
  • λ = wavelength in metres (m)

Evaluating the Experiment

Systematic Errors:

• It can be difficult to identify the wavefronts while they are moving
  • Use a stroboscope (flashing light) matched to the same frequency of the waves, this will be indicated by the waves appearing to be stationary
  • The frequency can be read from the frequency setting of the stroboscope, and the wavelength will be easier to determine while the waves appear still

Random Errors:

• To improve the accuracy of the wavelength measurement in the ripple tank:
  • Measure across a number of waves (e.g. 5 of them) and then divide the distance by the number of waves

• To improve the accuracy of the frequency measurement in the ripple tank:
  • Measure across a longer time period (e.g. a minute) and then divide the number of waves by the time

Safety Considerations

• Care should be taken when working with water and electricity in close proximity
  • Carelessness could lead to electric shock

• Make sure to stand up during the whole experiment, to react quickly to any spills
• No food or drink should be consumed near the experiment 
• If using strobe lighting to see the wavefronts more clearly, ensure no one in the room has photosensitive epilepsy