Not given in the exam, recall it.
What it is for
Wave speed is used in Waves (SPM Physics Form 4). Keep the SI units in every line of working. Waves
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| v | wave speed | m s⁻¹ |
| f | frequency | Hz |
| λ | wavelength | m |
Rearrangements
f = v / λλ = v / f
Worked example
Common trap
Understanding the wave equation
The wave equation v = fλ links how fast a wave travels to its frequency and wavelength. v is the wave speed in metres per second (m s⁻¹), f is the frequency in hertz (Hz), and λ is the wavelength in metres (m).
Frequency counts how many complete waves pass a point each second, and wavelength is the length of one complete wave. Multiplying them gives the distance the wave advances each second, which is its speed.
For a given medium the speed is fixed, so frequency and wavelength are inversely related: a higher frequency goes with a shorter wavelength.
To find frequency use f = v / λ; to find wavelength use λ = v / f. Convert every length to metres and keep frequency in hertz before substituting.
A worked example, step by step
A sound wave of frequency f = 500 Hz travels through air at v = 340 m s⁻¹. Find its wavelength.
Rearrange the wave equation to make λ the subject:
λ = v / f = 340 m s⁻¹ / 500 Hz = 0.68 m.
Because 1 Hz = 1 s⁻¹, the units work out as (m s⁻¹) ÷ (s⁻¹) = m, so the answer is a length: λ = 0.68 m (2 significant figures). This is a sensible wavelength for an audible sound.
If the frequency were doubled to 1000 Hz in the same air, the wavelength would halve to 0.34 m because the speed stays fixed by the medium.
Common mistakes and how this is tested
The trap examiners set is a unit mismatch: a wavelength given in centimetres or millimetres must be changed to metres first. For example λ = 40 cm becomes 0.40 m.
Other common mistakes: dividing when you should multiply (v = fλ is a product); thinking a higher frequency travels faster in the same medium (the speed is fixed by the medium, so a higher frequency simply gives a shorter wavelength); and confusing period with frequency.
Under calculate show v = fλ, substitute with units and state the unit of the answer. Under explain you may need to say why the wavelength shortens when frequency rises at constant speed.
Under define, state what wavelength and frequency mean.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)