From wavelength to energy
The calculation uses vacuum wavelength λ, optical frequency ν and the defining SI values of h and c. Multiply the single-photon energy by NA to express it per mole of photons.
ν̃ / cm⁻¹ = 10⁷ / (λvac / nm)
Photon rate = Poptical / E
The photon-rate extension assumes monochromatic light at the entered wavelength. For a broad source, calculate the spectral integral ∫Pλλ/(hc) dλ instead of treating the nominal LED wavelength as exact. Incident power does not account for reflection, transmission, absorption or illuminated area.
Worked example
450 nm → 2.75520 eV per photon → 265.837 kJ per mole of photons.
At 100 mW incident monochromatic power: 2.26535 × 10¹⁷ photons s⁻¹.
Common questions
Can I use the wavelength measured inside a medium?
The wavelength field means vacuum wavelength. If the in-medium wavelength and phase refractive index are known, first use λvac = nλmedium. Frequency, and therefore photon energy, does not change merely because light enters a stationary medium.
Does this calculate an apparent quantum yield?
No. That also requires the reaction-product amount or rate and a declared photon-counting convention. This page calculates incident photon rate, not absorbed photon rate or catalytic efficiency.
Sources and conventions
Method version 1.0.0 · 19 September 2026. Display precision is not measurement accuracy. Read the full scope and validation limits.