Why measure the length of 10 waves rather than 1?

It reduces the uncertainty in the wavelength

Why is a long distance used when timing a sound in air?

So the time is long compared with the reaction-time error

Core practical: measuring waves in liquids and solids

Core practical 4.17 measures the speed, frequency and wavelength of waves in a ripple tank and in a solid rod. You also need to measure the speed of sound in air.

GCSE · 1PH0 · 4.7, 4.17
About 16 minutes

Practise this lesson free (6 questions)

Ripple tank (waves in a liquid)

A vibrating bar makes straight ripples. A lamp above shines through the water so the waves show as bright and dark lines on paper below.
Wavelength: measure across 10 waves with a ruler (or on a photo) and divide by 10. This reduces the uncertainty.
Frequency: count the waves passing a point in 10 s and divide by 10 (or read the signal generator).
Speed: v = f × λ, or time one wave travelling a measured distance: v = x ÷ t.

Waves in a solid rod

Hang a metal rod horizontally and hit one end with a hammer.
A microphone connected to a frequency app or oscilloscope measures the frequency of the sound.
For the main note, the wavelength is twice the length of the rod: λ = 2 × L.
v = f × λ
Example: a 1.0 m steel rod gives 2500 Hz. λ = 2.0 m, so v = 2500 × 2.0 = 5000 m/s.

Speed of sound in air

Two people stand a measured distance apart (for example 100 m on a field). One bangs two blocks together; the other starts a stopwatch on seeing the blocks hit and stops it on hearing the sound.
v = distance ÷ time
Repeat and find a mean; reaction time causes errors, so use a long distance. A better method uses two microphones a measured distance apart connected to a timer or oscilloscope.

Worked example

In a ripple tank, 10 waves measure 15 cm and 60 waves pass a point in 10 s. Find the wave speed in m/s.

λ = 15 ÷ 10 = 1.5 cm = 0.015 m
f = 60 ÷ 10 = 6 Hz
v = 6 × 0.015 = 0.09 m/s

Answer: 0.09 m/s

Key idea

Ripple tank: measure 10 wavelengths and divide by 10; count waves per second for frequency; v = fλ. Metal rod: measure frequency with a microphone; λ = 2 × rod length. Sound in air: v = distance ÷ time over a long distance.

Key words

Frequency: The number of waves passing a point each second. Unit hertz (Hz). f = 1 ÷ T.
Wavelength: The distance from one point on a wave to the same point on the next wave, for example crest to crest.
Time: 1 hour = 3600 s. Convert minutes and hours to seconds before using P = E ÷ t or Q = I × t.
Generator: A device that uses electromagnetic induction to transfer energy from a kinetic store to an electrical supply.

All GCSE and International GCSE Physics key terms and definitions →

Quick check

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Extra practice

2 more questions

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Eight waves in a ripple tank measure 20 cm. What is the wavelength in metres?
Show the answer
Answer: 0.025 m
20 ÷ 8 = 2.5 cm = 0.025 m
A 0.50 m metal rod gives a note of frequency 5000 Hz. Calculate the speed of sound in the rod, using λ = 2 × length.
Show the answer
Answer: 5000 m/s
λ = 2 × 0.50 = 1 m. v = f × λ = 5000 × 1 = 5000 m/s

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Practise this lesson free (6 questions)All GCSE and International GCSE Physics lessons

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Sources and references

Checked by the GCSE-ready editorial team, 17 October 2026. Last updated 2 October 2026. Spotted a mistake? Tell us and we will fix it.

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