Wavelength and frequency
3 marks · NormalA student states, 'If the frequency of water waves in a ripple tank is doubled, the speed of the waves will also double.'
Explain whether this statement is correct.
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The statement is incorrect. In a ripple tank, the wave speed is determined by the properties of water and usually remains the same. When frequency increases, the wavelength decreases so that the product of frequency and wavelength stays constant (since v = fλ).
Also accept: Doubling frequency decreases wavelength not speed; Wave speed remains constant while wavelength changes
Common mistakes: Speed always increases with frequency
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Marking points
B1 · 1 mark · States the speed does not double or does not increase.
M1 · 1 mark · Explains that wave speed in water is usually constant (depends on medium).
A1 · 1 mark · States that as frequency increases, wavelength decreases so that v = fλ is maintained.
Worked solution
The statement is not correct. In a ripple tank, the speed of the water waves is set by the properties of water and stays about the same.
Doubling the frequency makes the wavelength halve, so that v = fλ remains true. The wave speed does not double as the frequency increases; wavelength simply gets shorter.