Edexcel IGCSE Physics · Spec 3.8
Investigating Waves
Required practical: investigating waves in a ripple tank and on a string.
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Investigating Waves
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Explained
Measuring waves you can see
Wave speed equals frequency times wavelength, so measuring a wave speed means measuring those two things separately. The practical exists because each is awkward in its own way: the frequency is easy to set but hard to see, and the wavelength is easy to see but hard to measure while it moves.
Water waves in a ripple tank
A vibrating dipper connected to a signal generator makes straight ripples. The frequency is whatever you set on the generator, so it is known rather than measured.
For the wavelength, place a ruler on the bench beneath the tank and let the ripples cast shadows on paper below. Photograph or freeze the pattern, then measure across as many wavelengths as you can, ten if possible, and divide by that number.
Measuring ten and dividing is the important step. A single wavelength is a centimetre or two, and the uncertainty in reading a ruler is the same whether you measure one or ten, so spreading it across ten reduces the percentage uncertainty tenfold.
Then wave speed is the frequency times that wavelength.
Waves on a string
A string under tension is driven by a vibration generator at one end and passes over a pulley to a hanging mass at the other.
Adjust the frequency until a stationary pattern appears with clear still points, called nodes. Measure the distance between nodes: that is half a wavelength, so double it.
Read the frequency off the generator, multiply, and you have the speed. Repeat at several frequencies, and the speed should come out the same each time, because it depends on the string's tension and mass rather than on how fast you drive it.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme for measuring a wavelength from a scaled image awards one mark for a measurement within a stated range and a second for using the scale factor correctly to convert it. It allows an error carried forward provided it is clear that a scale factor is being used.
Showing the conversion is therefore worth a mark on its own. A measured length written down with no indication of how it became a real distance cannot earn it.
On the calculation itself, the same paper gives a mark for converting centimetres to metres, allowing the division by 100 seen anywhere in the working, and states that not converting caps the question at two marks out of three.
On a question about a wave from a moving dipper, the mark scheme credits the frequency being greater, the wavefronts being closer together, the wavelength decreasing and the wave speed not changing. Its note then says to ignore the speed of the dipper.
That last instruction is the subtle one. How fast the dipper moves across the tank changes the pattern you observe, but it does not change how fast the waves travel through the water. Wave speed depends on the medium, not on the source.
Sources of error, and how to reduce them
Ripples are faint and move quickly, so the wavelength is hard to read. A stroboscope flashing at the wave frequency freezes the pattern and makes it measurable, and a photograph does the same permanently.
Reflections from the tank walls interfere with the incoming waves and blur the pattern. Sloping absorbent edges reduce them.
And the water depth must stay constant, because the speed of a water wave depends on depth. A tank that is not level gives waves that change speed as they travel, which is itself the standard demonstration of refraction.
Spec 3.8
What you need to know
- Measure the frequency and wavelength of waves
- Find the wave speed from those measurements
- Describe doing this with a ripple tank and with a string
Active recall
Quick check
Answer each question before opening the answer.
How can you find the speed of water waves in a ripple tank?
Measure the frequency and the wavelength and use v = f × λ.
How do you measure the frequency of a wave?
Count the number of waves passing a point each second (or use f = 1 ÷ T).
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