The formula
v = λ · f
| Symbol | Quantity | Unit |
|---|---|---|
| v | Wave speed | m/s |
| L | Wavelength | m |
| f | Frequency | Hz |
What it means
Every wave obeys the same relationship: how far one cycle stretches, multiplied by how many cycles pass per second, gives the speed. It holds for sound, light, water waves and everything else that oscillates.
How to work it out by hand
- Take the wave speed for the medium
- Divide by the frequency for the wavelength
- Or divide by the wavelength for the frequency
What is worth knowing
The speed is a property of the medium, not of the wave, which is why the wavelength changes when light enters glass but the frequency does not — and that change of wavelength at constant frequency is exactly what bends the light. Sound travels at about 343 m/s in air and around 1500 m/s in water, so the same note has a wavelength four times longer underwater. For light in a vacuum, c is 299,792,458 m/s exactly, because the metre is now defined from it.