PAG4: Measuring Waves
Wavelength is the distance along the direction of travel for one full cycle, such as crest to the next crest. Frequency counts cycles passing a point per second; period is the time for one cycle. Measure several complete spacings or cycles and divide by the number of intervals to reduce the fractional effect of a single reading uncertainty. Six crest marks define five adjacent crest-to-crest intervals, not six.
Before you start
- Use your teacher’s tier checklist: this overview can include Higher-only material, and is not a tier-specific revision list.
- OCR Gateway A separate GCSE Physics J249. P9 groups the practical techniques into PAG1–8; the activities here are suitable examples, and centres can choose alternatives covering the required apparatus and techniques.
- Distinguish frequency, period, wavelength and amplitude.
- Use the centre’s approved supervised method and equipment limits. These notes prepare you to explain and evaluate practical work; they do not replace carrying it out and recording the evidence.
Watch, pause, try
Video by Science Shorts. This overview supports wave-measurement reasoning. Solid-wave apparatus differs by course; Pearson’s written guide and linked solid-wave demonstration explicitly develop the metal-rod method.
Viewing focus: Choose apparatus for wavelength, frequency and propagation time
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Explain how you would avoid two errors when measuring wavelength from a projected ripple image.
Show answer and reasoning
Calibrate the projection scale to recover actual lengths, and divide a span across several crests by the number of complete crest-to-crest intervals rather than the number of marks.
After watching 2: Why is a standing-wave pattern’s visible length insufficient by itself to calculate wavelength?
Show answer and reasoning
The number of loops or the identified vibration mode is also required. Each adjacent-node spacing is half a wavelength; different modes fit different wavelengths into the same length.
What you'll be able to do
- Choose apparatus for wavelength, frequency and propagation time
- Calculate wave speed with compatible measurements
- Evaluate projection scale, interval counting and solid-wave methods
Separate wavelength, frequency and speed
Wavelength is the distance along the direction of travel for one full cycle, such as crest to the next crest. Frequency counts cycles passing a point per second; period is the time for one cycle. Measure several complete spacings or cycles and divide by the number of intervals to reduce the fractional effect of a single reading uncertainty. Six crest marks define five adjacent crest-to-crest intervals, not six.
For one wave in a consistent medium, v = fλ. A distance divided by the time taken by a recognisable wave disturbance provides another speed estimate. Do not multiply frequency from one setting by wavelength from another. Changing water depth can change wave speed, so record depth and keep the relevant conditions comparable when testing relationships.
Choose a workable ripple-tank observation
A ripple tank makes water-wave patterns visible, often by projecting shadows onto a screen. Measure the pattern scale against a known distance in the same projection arrangement: a magnified shadow is not automatically the actual wavelength. A still image or suitable synchronised strobe can help identify crests, but a visually stationary pattern does not mean the water-wave frequency is zero.
Use a suitable low-amplitude wave pattern and identify a region where the crests are clear. Count cycles over an appropriately long timed interval or use a correctly interpreted drive-frequency reading. Keep the tank level and minimise unwanted reflections where appropriate. Explain why the selected apparatus can resolve the frequency and distances being measured; merely naming a stopwatch is insufficient for a rapidly vibrating system.
Use a solid method that matches the qualification
A wave in a stretched string can be driven by a signal generator, with its frequency read from the source and its wavelength inferred from a clear pattern. For a standing wave, neighbouring nodes are separated by half a wavelength, so the whole visible length is not automatically one wavelength. State the boundary conditions and the number of loops before using the length.
A longitudinal vibration in a suitable metal rod is a different solid-wave method. In an appropriate fundamental longitudinal mode with a central node and free ends, rod length corresponds to half a wavelength, so λ = 2L. A microphone and suitable frequency display can identify a resonance, but selecting a harmonic instead of the fundamental changes the interpretation. Use the centre’s approved support/excitation arrangement. Pearson explicitly requires the metal-rod context; a string demonstration alone does not fulfil that named subpart.
Worked applications
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Example 1Six successive crest marks span 15.0 cm in a calibrated ripple pattern. The frequency is 12.0 Hz. Find wavelength and wave speed.
Example 2A string has three equal standing-wave loops along 0.90 m. Its drive frequency is 20.0 Hz. Find wavelength and speed.
Example 3A 1.20 m rod vibrates in its identified fundamental longitudinal mode at 2100 Hz, with a central node and free ends. Estimate longitudinal wave speed.
Common mistakes to avoid
Key takeaways
- You can choose apparatus for wavelength, frequency and propagation time.
- You can calculate wave speed with compatible measurements.
- You can evaluate projection scale, interval counting and solid-wave methods.
Apply it in an exam
Question 1: A 1.20 m rod vibrates in its identified fundamental longitudinal mode at 2100 Hz, with a central node and free ends. Estimate longitudinal wave speed.
Show answer and reasoning
About 5.04 × 10³ m s⁻¹; this is original illustrative data, not a reference value for an unidentified metal.
- The stated mode has λ = 2L = 2.40 m.
- v = fλ = 2100 × 2.40.
- The result depends on identifying that mode correctly.
Explore further
- OCR J249 official specification (opens in a new tab)
Check the assessed scope and practical or mathematical requirements.
- Read or explore this topic (opens in a new tab)
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
- Measuring waves in a solid — Science School (opens in a new tab)
A verified metal-rod example supporting the solid-wave technique. Pearson numbering in the video does not change the OCR PAG4 reference.
Return to this tomorrow
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
- What does a synchronised stationary-looking ripple image establish?
- For the specified free-ended rod fundamental with a central node, what is the wavelength?
Test yourself
Answer a few questions, then review anything you missed.
Keep practising
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.