How it works
All electromagnetic waves travel at the speed of light, c = 299,792,458 m/s, in vacuum. Wavelength and frequency are inversely related: λ = c ÷ f.
Light also comes in photons whose energy is proportional to frequency: E = hf, with Planck’s constant h = 6.62607015 × 10⁻³⁴ J·s. Both c and h are exact by definition in the SI.
A handy shortcut: photon energy in eV ≈ 1239.84 ÷ wavelength in nm. Green light at 500 nm carries about 2.48 eV per photon.
Formula
λ = c ÷ f E = h × f = h × c ÷ λ E(eV) ≈ 1239.84 ÷ λ(nm)
Questions people ask
What wavelengths are visible?
Roughly 380 to 750 nm, from violet to red. Sensitivity peaks near 555 nm (green) in bright light.
Why does UV damage skin but radio waves do not?
UV photons carry 3–12 eV each, enough to break chemical bonds in DNA. Radio photons carry millionths of an eV, far too little, no matter how many there are.
What is a wavenumber?
The number of waves per centimeter, 1/λ. Spectroscopists use it because it is proportional to energy. 500 nm equals 20,000 cm⁻¹.