Physics · Light and quanta
Photon energy calculator.
Enter any one of wavelength, frequency or energy. Get the other two, in electronvolts and joules, using the exact SI values of h and c.
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Photon energy formula
A photon of frequency f carries energy E = hf, and since f = c/λ, E = hc/λ. Both h (6.62607015×10−34 J s) and c (299,792,458 m/s) are exact in the SI, as is the electronvolt (1 eV = 1.602176634×10−19 J). A handy result is hc ≈ 1239.84 eV nm, so E(eV) ≈ 1239.84 / λ(nm).
Worked example. A 500 nm photon: f = c/λ = 5.996×1014 Hz (599.6 THz); E = hf = 3.973×10−19 J = 2.480 eV. Multiplying by Avogadro's number gives about 239.3 kJ per mole of photons.
Frequently asked questions
What is the formula for photon energy?
E = h f = h c / lambda, with h = 6.62607015e-34 J s and c = 299,792,458 m/s.
How do I convert wavelength to eV quickly?
Divide 1239.84 by the wavelength in nanometres. 500 nm gives 2.48 eV.
How do I convert nm to THz?
Frequency = c / wavelength. For 500 nm that is 599.6 THz. Shorter wavelength means higher frequency.
Are the constants exact?
Yes. Since the 2019 SI redefinition h, c and e have exact values, so the only rounding is in the display.
Does the medium change the photon energy?
No. Frequency and energy stay fixed; the wavelength in a medium is the vacuum wavelength divided by the refractive index.
Sources
Checked
- NIST CODATA: Planck constant h (exact, 2022 values)
- NIST CODATA: speed of light in vacuum c (exact)
- NIST CODATA: elementary charge e (exact; sets the electronvolt)
- NIST CODATA: Avogadro constant (exact)
- OpenStax University Physics Vol. 3: Key equations, ch. 6 (photon energy E = hf)
NIST lists h, c and e as exact. E = hf is given by OpenStax. The 1239.84 eV nm figure and kJ per mole (times Avogadro constant 6.02214076e23, exact) are computed from those constants. Runs entirely in your browser. Nothing you type is sent or stored.
Scope
Physics reference calculator. Not for laser safety or exposure calculations.