A photon's energy depends only on its frequency or wavelength: E = hf = hc/λ. This calculator takes a wavelength or frequency and returns the energy in joules and electronvolts, the matching frequency or wavelength, and the energy per mole of photons.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: 2019 SI constants, recomputed in code.
The formula
Energy rises with frequency and falls with wavelength. Dividing joules by the electronvolt (1.602177×10⁻¹⁹ J) gives energy in eV, the natural unit in atomic physics. Multiplying a single photon's energy by Avogadro's number gives kJ per mole, which links photon energy to chemistry. The shortcut E(eV) ≈ 1239.84 ÷ λ(nm) bundles all the constants for visible-light estimates.
Worked examples
500 nm (green light):
650 nm (red light):
0.1 nm (X-ray):
Per mole, the 500 nm photon carries about 239 kJ/mol — comparable to many chemical bond energies, which is why visible and UV light can drive photochemistry. The X-ray, at 12.4 keV, is thousands of times more energetic per photon.
Frequently Asked Questions
E = hf, or E = hc/λ since f = c/λ. h is Planck's constant.
Shortcut: E(eV) ≈ 1239.84 / λ(nm). 500 nm ≈ 2.48 eV.
≈ 3.97×10⁻¹⁹ J = 2.48 eV, f ≈ 6.00×10¹⁴ Hz (green light).
E = hc/λ — energy is inversely proportional to λ. Halve λ, double E.
Photon energy × Avogadro's number. 500 nm ≈ 239 kJ/mol.