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Measuring ripple-tank frequency and wavelength
A student made water waves in a ripple tank.
Describe how the frequency and wavelength of the water waves in the ripple tank can be measured accurately.
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Marking points
- 1 Describe a valid way to measure wavelength, such as measuring a known length across the wave pattern and dividing by the number of complete waves, or measuring the distance between wave fronts.
- 2 Include an accuracy improvement for wavelength, such as using a metre rule with a photograph or measuring across several waves rather than only one.
- 3 Describe a valid way to measure frequency by counting waves passing a fixed point and timing them.
- 4 Calculate frequency as number of waves divided by time, or use slow-motion video to count waves passing a point in a known time.
- 5 Present the frequency and wavelength methods in a logical sequence that would produce valid measurements.
Full-mark answer
For the wavelength, put a metre rule beside the screen perpendicular to the wave fronts and take a photograph of the wave pattern. Measure the distance covered by several complete waves and divide by the number of waves to find the wavelength. For the frequency, choose a fixed point and count how many waves pass that point in a measured time using a stop clock; frequency is the number of waves divided by the time. A video could be replayed in slow motion to make the counting more accurate.
Why this answer loses marks
Measure the distance between two waves with a ruler and count the waves.
This gives some wavelength idea but does not describe timing waves passing a fixed point to measure frequency.