Wavelength to Energy Calculator
Convert between wavelength, frequency, wavenumber and energy — the four ways spectroscopists label the same photon.
Four names for one photon, converted.
How the wavelength to energy calculator works
A photon can be described by wavelength (nm), frequency (Hz), wavenumber (cm⁻¹) or energy (eV or J), and different fields prefer different ones: optics uses nanometers, infrared spectroscopy wavenumbers, X-ray work kiloelectronvolts.
All four are linked by E = hf = hc/λ and ν̃ = 1/λ. The useful shortcut is E(eV) = 1239.84 ÷ λ(nm).
Formula: E(eV) = 1239.84 / λ(nm); ν̃ = 10⁷/λ(nm) cm⁻¹
Worked examples
| Inputs | Energy | Note |
|---|---|---|
| 1,000 nm | 1.239842 eV | 1.2398 eV, 10,000 cm⁻¹ |
| 532 nm green laser | 2.33053 eV | 2.331 eV |
| 10 µm thermal infrared | 0.123984 eV | 0.124 eV, 1,000 cm⁻¹ |
FAQFrequently asked questions
What is the 1239.84 rule?
E in electronvolts equals 1239.84 divided by the wavelength in nanometers. It is the single most useful constant in spectroscopy.
What is a wavenumber?
Reciprocal centimeters — how many wave cycles fit in a centimeter. Infrared spectroscopy uses it because it is proportional to energy and gives convenient numbers.
Why do different fields use different units?
Convenience of magnitude. Optical work gives clean numbers in nm, IR in cm⁻¹, X-ray in keV.
What is the equivalent temperature?
E/k — the temperature at which thermal energy matches the photon. It shows which transitions are accessible at a given temperature.
How much energy per mole?
Multiply the photon energy by Avogadro's number: 1 eV per photon is 96.485 kJ/mol, which is why visible light drives chemistry at all.
Where these figures come from
- NIST — CODATA 2018 fundamental physical constants — G, g₀, R, c
- NIST Special Publication 811 — Guide for the use of the International System of Units — unit conversions
- The Engineering ToolBox — material properties — specific heats, expansion coefficients, densities
- National Institute of Standards and Technology — the US measurement authority
Last checked: September 2026. Constants are the CODATA 2018 values; formulas are the standard textbook forms.