The energy of a single photon depends solely on its wavelength: the shorter the wavelength, the more energetic the photon. This calculator takes a wavelength in nanometres and returns the photon energy in joules and in electronvolts.
The quantity explains why UV radiation acts differently from visible light. A photon at 254 nm carries about 4.9 eV, above the binding energy of many organic bonds – it can break molecules directly. A photon at 400 nm carries only about 3.1 eV. It is exactly this threshold that decides whether a photoinitiator responds, whether a semiconductor transition is excited, or whether a photocatalytic reaction starts. If you know a band gap in eV, the calculator tells you the longest wavelength you may use for it.
The calculation applies to photons in vacuum or air; in a medium with refractive index n the wavelength changes but the photon energy does not. The constants follow CODATA 2018.
Photon energy leads directly to photon flux – the number of photons per second. That calculator is embedded in the photochemistry page, where the question arises. All radiometric calculators are collected in the radiometry calculator.
Fundamentals
Photon energy follows from the Planck–Einstein relation E = h · c / λ, with the Planck constant h = 6.62607015 · 10⁻³⁴ J·s and the speed of light c = 299 792 458 m/s (both exactly defined since the SI revision). Entering λ in metres gives E in joules. For electronvolts, divide by the elementary charge, which yields the convenient short form E[eV] = 1239.842 / λ[nm].
| Wavelength | Energy |
|---|---|
| 254 nm (low-pressure mercury) | 4.88 eV |
| 265 nm (UVC LED) | 4.68 eV |
| 280 nm (UVC LED) | 4.43 eV |
| 365 nm (UVA LED) | 3.40 eV |
| 405 nm | 3.06 eV |
Worked example for 365 nm: E = 1239.842 / 365 = 3.397 eV, equal to 5.44 · 10⁻¹⁹ J.
The formula describes a single photon of a single wavelength. A medium-pressure mercury lamp emits many lines, an LED a band of typically 10–15 nm full width at half maximum — here the centroid wavelength is an approximation, and accurate work requires integrating the spectrum. Photon energy also says nothing about the amount of radiation; that requires irradiance or photon flux.
Frequently asked questions
With E = h · c / λ. In practice the short form E[eV] = 1239.842 / λ[nm] is usually enough: 365 nm gives 3.40 eV, 254 nm gives 4.88 eV.
With the same short form. The relation is reciprocal, not linear — half the wavelength means twice the energy.
Because the individual photon is more energetic. At 254 nm the photon energy exceeds the binding energy of many organic bonds, at 365 nm it does not. UV-C therefore acts directly on DNA, while UV-A acts predominantly through intermediate steps. The classification of ranges is described under spectral ranges and standards.
λ = 1239.842 / 3.2 ≈ 387 nm. For reliable excitation a slightly shorter wavelength is chosen in practice, around 365 or 375 nm.
No. Inside a medium the wavelength shortens by the refractive index, but the frequency and therefore the energy stay the same. Formulas must use the vacuum wavelength.
It decides whether a photoinitiator is excited at all. If the wavelength does not match the absorption range of the initiator, even a high dose will not help. More on this under photoinitiators & UV absorbers.
Author: Dr. Mark Paravia
Dr.-Ing. Mark Paravia is the managing director of Opsytec Dr. Gröbel GmbH in Ettlingen and heads the accredited calibration laboratory. Following his research on pulsed xenon excimer discharges at the Institute of Lighting Technology at KIT, his current focus is on optical radiation measurement technology. He is vice-chair of the DIN Standards Committee FNL 7 “Optical Radiation” and a member of the DVGW Project Group on UV Disinfection.
