How do you do the waves required practical?

What does the waves practical investigate?

It measures the frequency, wavelength and speed of waves in two places: water waves in a ripple tank, and waves on a stretched string or elastic cord. Part of the job is judging which equipment gives good measurements, because both kinds of wave move too fast to measure by eye without some help.

It is a required practical on AQA GCSE Physics and on AQA GCSE Combined Science: Trilogy.

What equipment do you need?

There are two set-ups: a ripple tank for water waves, and a vibrating string for waves in a solid.

• A ripple tank with a wave generator (a motor-driven dipper), a lamp above it and a power supply
• A large sheet of white paper under the tank, a metre ruler and a stopwatch
• A stroboscope or a camera, if you have one, to freeze the pattern
• A vibration generator connected to a signal generator
• A string or elastic cord, a pulley, a mass hanger and slotted masses

What is the method?

Do the ripple tank first, then the string. For both, measure across as many waves as you can and divide.

• Fill the ripple tank with a shallow, even depth of water, and check the tank is level so the depth is the same everywhere.
• Switch on the lamp and the wave generator. Adjust the frequency until clear, evenly spaced waves show on the paper below.
• Lay a metre ruler on the paper at right angles to the waves. Measure across 10 waves and divide by 10 to get one wavelength.
• Count the waves passing a mark on the paper in 10 seconds, and divide by 10 to get the frequency.
• For the string, tie one end to the vibration generator, run it over the pulley and hang masses on the other end to keep it tight.
• Switch on the signal generator and adjust the frequency until the string forms a steady pattern of loops.
• Measure across as many loops as you can and divide by the number of loops. Double that to get the wavelength, and read the frequency from the signal generator.

What are the variables?

This practical is more about measuring well than testing a relationship. If you change the frequency, the wavelength changes in response, while the things that set the wave speed stay fixed.

How do you work out and present the results?

Use the wave equation v = f λ: wave speed (m/s) = frequency (Hz) × wavelength (m). Change wavelengths from centimetres to metres before you multiply.

Ripple tank example: 5 waves span 20 cm on the paper, so λ = 4.0 cm = 0.040 m. You count 30 waves passing a mark in 10 s, so f = 3.0 Hz. Then v = 3.0 × 0.040 = 0.12 m/s.

String example: the signal generator is set to 40 Hz and 4 loops cover 1.20 m. One loop is 0.30 m, so λ = 0.60 m and v = 40 × 0.60 = 24 m/s.

If you repeat the string experiment at several frequencies, the wavelength gets shorter as the frequency goes up, but f × λ stays about the same. The speed depends on the string and how tight it is, not on the frequency. A graph of wavelength against 1 ÷ frequency is then a straight line through the origin, and its gradient is the wave speed.

How do you make it accurate and safe?

Waves move too quickly to measure one at a time, so measure across many and divide. A stroboscope helps: when its flash rate matches the frequency of the waves, the pattern looks frozen and is far easier to measure. A photo of the pattern with a ruler in the picture does the same job. On the string, adjust the frequency until the loops are sharp and still before you measure.

On safety: keep the power supply and plugs away from the water, and mop up spills at once. The lamp gets hot. Check with your teacher before using a strobe, because flashing light can trigger seizures in people with photosensitive epilepsy. For the string, clamp everything firmly and keep your feet clear of the hanging masses.

What do exam questions ask about it?

You might be asked how to measure the wavelength or the frequency accurately, and why measuring across several waves helps. Calculations with v = f λ come up often, usually with a unit to convert. Longer questions can ask you to describe the whole method.

Here is one we wrote in the exam style: "A student sees 5 loops on a string 1.50 m long when the frequency is 50 Hz. Calculate the speed of the waves on the string. (3 marks)" Each loop is 0.30 m, so the wavelength is 0.60 m, and v = 50 × 0.60 = 30 m/s.

Common mistakes

• Counting lines instead of gaps. From the first bright line to the eleventh is 10 wavelengths, not 11.
• Treating one loop on the string as a whole wavelength. A loop is half a wavelength.
• Leaving the wavelength in centimetres. For a speed in m/s, the wavelength must be in metres.
• Measuring a single wave. Measuring across 10 and dividing shares the measuring error between 10 waves, so each wavelength is far more accurate.