Wavelength to Frequency Converter

Green laser pointers are sold by wavelength, Wi-Fi routers by frequency, and piano tuners talk in hertz while acoustic engineers think in metres. They are all describing the same thing. Type any one of wavelength, frequency or period, for light and radio or for sound, and this converter gives you the other two at once.

Alongside them it shows the wave speed, angular frequency, angular wavenumber and, for light, the photon energy and spectroscopic wavenumber. A strip along the bottom places the wave on the electromagnetic spectrum or the range of human hearing. The wavelength and frequency are also listed in seven units each, from picometres to kilometres and from hertz to exahertz.

Worked Example: A 532 nm Green Laser

The page opens with Light & radio selected, a wavelength of 532 nm and a vacuum medium. It reports:

• Frequency: 563.52 THz, with a period of 1.77456 fs
• Photon energy: 2.33053 eV, and a wavenumber of 18,797 cm⁻¹
• The band reads Visible light (green), with a colour swatch beside it
• Switch the medium to Water and the frequency stays put while the wavelength inside drops to 399.1 nm

Load Sample moves on to 2.4 GHz Wi-Fi (12.4914 cm), concert A at 440 Hz in 20 °C air (78.0033 cm), 5 MHz medical ultrasound in soft tissue (308 µm), FM radio at 100 MHz, a 1.5 m wave in fresh water and 1,550 nm telecom light inside a silica fibre.

How the Math Treats Light, Sound and the Medium

Every result comes from v = f × λ and T = 1 ÷ f. For light, v is 299,792,458 m/s divided by the refractive index. The wavelengths you type are vacuum wavelengths, the figure laser datasheets and spectral tables quote. A medium never changes the frequency; it only squeezes the wavelength inside it to λ₀ ÷ n. Photon energy is E = hf, using the exact SI values of the Planck constant and the electronvolt.

Sound has no vacuum wavelength, so its wavelength is always the one in the medium you pick. In air the speed follows the temperature, 331.3 × √(1 + T ÷ 273.15) m/s, which gives 343.215 m/s at 20 °C. Two details are easy to miss. Change I know the from wavelength to frequency and the answer you already have is written into the box in a sensible unit, so nothing goes blank. And for audible sound the tool names the nearest equal-tempered note and how many cents sharp or flat it is: the 1.5 m wave in water comes out at 987.333 Hz, which is B5, 1 cent flat.

Precision and Where the Presets Stop Being Exact

Results are shown to six significant figures and switch to × 10ⁿ notation below 0.0001 and from one billion up. Wave speed is shown to nine, so the speed of light appears as its exact defined value. The input box takes plain decimals, 4.74e14 and 4.74 × 10^14 alike.

The medium presets are approximations. Refractive indices are quoted near 589 nm, but water bends violet light slightly more than red, so treat the in-medium wavelength as good to about 1%. The air formula assumes dry air, and humidity raises the real speed of sound a little. Where it matters, choose Custom index or Custom speed and type the measured value. An index below 1 is refused with: “Refractive index must be a number of 1 or more. Light is never faster than in a vacuum, where n = 1.”