Wavelength to Frequency Converter
Convert a wavelength into a frequency. Enter the wavelength in nm, µm, mm, cm, or m, choose the medium — light in vacuum (default), water, or typical glass, or sound in air — and get f = v/λ scaled from Hz to THz, the photon energy in eV, and where it sits in the spectrum.
Example: with Wavelength 550 · Wavelength unit nm (nanometers) · Wave and medium Light in vacuum (n = 1) → Frequency f: 545.1 THz.
- Photon energy2.254 eV per photon
- Where it sits in the spectrumVisible — green (vacuum λ = 550 nm)
Computed by the calculator below using its default values. Change any input to see your own numbers.
f = v/λ, with v = c/n for light (c = 299,792,458 m/s exactly). Photon energy E = h·f using h = 6.62607015×10⁻³⁴ J·s (exact, 2019 SI).
Frequency is the wave's fingerprint
Dividing the wave speed by the wavelength gives the frequency: f = v/λ. For light in vacuum that speed is exactly 299,792,458 m/s, so green light at 550 nm oscillates at about 545 THz — 545 trillion cycles every second. Frequency is the property that never changes as light crosses between materials, which is why physicists prefer it for identifying radiation. Wavelength is what stretches and compresses with the medium.
This converter also reports photon energy, because frequency and energy are the same information: E = h·f. At 545 THz each photon carries about 2.25 eV, enough to trigger the photoreceptors in your eye but far below the ~10⁵ eV of medical X-rays.
The in-a-medium subtlety
Enter 500 nm measured inside water and you do not get the frequency of 500 nm vacuum light. Light slows to c/1.333 in water, so a 500 nm in-water wave corresponds to 450 THz — the frequency of what would be 666 nm red light in vacuum. The color your eye perceives tracks frequency, not the local wavelength. That is the point of the medium selector: it converts using v = c/n and then reports the vacuum-equivalent wavelength so you can place the wave on the familiar spectrum.
How it’s calculated
f = v/λ. For light, v = c/n with c = 299,792,458 m/s (exact) and n = 1 (vacuum), 1.333 (water), or 1.5 (typical crown glass); for sound, v = 343 m/s (dry air, 20°C). Photon energy E = h·f with h = 6.62607015×10⁻³⁴ J·s and 1 eV = 1.602176634×10⁻¹⁹ J (both exact in the 2019 SI). Spectrum band assigned from the vacuum-equivalent wavelength c/f, using visible bounds of 380–750 nm.
Refractive indices are representative mid-visible values — real n varies with wavelength (dispersion), typically by a few percent across the visible band.
Visible spectrum by frequency
| Color | Vacuum wavelength | Frequency |
|---|---|---|
| Violet | 380 – 450 nm | 789 – 666 THz |
| Blue | 450 – 495 nm | 666 – 606 THz |
| Green | 495 – 570 nm | 606 – 526 THz |
| Yellow | 570 – 590 nm | 526 – 508 THz |
| Orange | 590 – 620 nm | 508 – 484 THz |
| Red | 620 – 750 nm | 484 – 400 THz |
Computed with f = c/λ from conventional visible-band boundaries; band edges are approximate — perception varies by observer.
Common mistakes
- Forgetting the unit prefix: 550 nm entered as 550 m is a 12-orders-of-magnitude error.
- Using c for light inside glass or water — divide by the refractive index first, or select the medium here.
- Assuming the color of in-medium light from its local wavelength; color follows frequency, so convert to the vacuum-equivalent wavelength before reading a color chart.
- Asking for photon energy of a sound wave — sound is a pressure wave, not electromagnetic radiation, so E = h·f does not apply.
Frequently asked questions
What is the wavelength to frequency formula?
f = v/λ. For light in vacuum, f = c/λ with c = 299,792,458 m/s, so f in THz = 299,792 / λ in nm.
What frequency is 550 nm light?
299,792,458 / 550×10⁻⁹ ≈ 5.45×10¹⁴ Hz, or 545 THz — green light near the peak sensitivity of human vision.
Does frequency change when light enters water or glass?
No. Frequency is set by the source and stays constant across boundaries; the speed drops to c/n and the wavelength shortens by the same factor. That is the common confusion this tool's medium option untangles.
How do I get photon energy from wavelength?
E = h·c/λ. In convenient units, E in eV ≈ 1240 / λ in nm, so 550 nm light carries about 2.25 eV per photon.
Can I convert sound wavelengths too?
Yes — choose sound in air and the tool uses 343 m/s. A 1 m sound wave is 343 Hz. Photon energy is not reported because sound is mechanical, not electromagnetic.