This photon energy calculator computes E = hf = hc/λ from wavelength or frequency, reporting energy in joules and electron-volts and placing the photon on the electromagnetic spectrum.
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E = hf: Light's Price Tag, Per Photon
The wave side is our Wavelength Calculator. Part of our Physics Calculators collection.
Worked Example (Verified)
- Green light, λ = 500 nm (defaults): f = c/λ = 5.996 × 10¹⁴ Hz → E = hf = 3.97 × 10⁻¹⁹ J = 2.48 eV — the energy currency photosynthesis and silicon solar cells trade in
- The eV shortcut worth memorizing: E(eV) ≈ 1240/λ(nm) — 1240/500 = 2.48 eV exactly as computed; UV at 250 nm doubles it to 4.96 eV, into bond-breaking territory
- Why microwaves (E ~ 10⁻⁵ eV) can't ionize anything no matter the wattage: photon energy is per-photon, set by frequency alone — intensity only adds more photons, each still too weak (the photoelectric lesson)
Photon Discipline
- Brightness ≠ energy per photon: a brighter red beam has MORE photons, each still 1.9 eV; a dim UV beam's photons each carry 4+ eV.
- eV is the native unit at atomic scales: ionization of hydrogen needs 13.6 eV → λ ≤ 91.2 nm (extreme UV) — one division tells you which light can do it.
- Photon momentum rides along: p = E/c = h/λ (basis of solar sails and optical tweezers).
- Per-mole photon energy (an “einstein”) at 500 nm is 239 kJ/mol — comparable to chemical bonds, which is why photochemistry happens in the visible/UV and not the radio.
Related Tools
- Photoelectric Effect Calculator — photons vs work functions
- De Broglie Wavelength Calculator — the matter-wave mirror
- Physics Calculators — the full collection
Frequently Asked Questions
How do you calculate photon energy?
E = hf = hc/λ, with h = 6.626 × 10⁻³⁴ J·s and c = 3.00 × 10⁸ m/s. For λ = 500 nm: E = 3.97 × 10⁻¹⁹ J = 2.48 eV. Enter wavelength in nm or frequency in Hz here and both units are reported.
What is the 1240 rule for photon energy?
E (in eV) ≈ 1240 / λ (in nm) — hc pre-multiplied and converted. 1240/500 = 2.48 eV for green light; 1240/91.2 ≈ 13.6 eV marks hydrogen's ionization wavelength. It's the fastest mental conversion in atomic physics.
Does brighter light mean more energetic photons?
No — intensity is photon COUNT per second; energy per photon is frequency alone. This is the photoelectric effect's central fact: below the threshold frequency, unlimited brightness ejects zero electrons.
How much energy is one mole of photons?
Multiply by Avogadro's number: at 500 nm, 3.97 × 10⁻¹⁹ J × 6.022 × 10²³ ≈ 239 kJ/mol — right in chemical-bond territory (C–C ≈ 348 kJ/mol), explaining why visible/UV light drives chemistry.
Which photons are dangerous?
Those above ~10 eV (λ < 124 nm: extreme UV, X-rays, gamma rays) can ionize atoms. Ordinary UV (3–10 eV) breaks some bonds and damages skin/DNA indirectly; visible and below are non-ionizing per photon.
Is this photon energy calculator free?
Yes — free, instant, no sign-up.