Photon Energy Calculator
Light in quanta: E = h·c/λ turns your wavelength into the energy of ONE packet — in joules, electron-volts, per mole — and counts how many photons a watt of that light spends per second.
Photon Energy Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Single-photon arithmetic — no material response
- Band edges are conventional, not sharp
In short: Green light at 550 nm carries E = h·c/λ = 3.612e-19 J per photon = 2.254258 eV — the exact constants h = 6.62607015e-34 J·s and c = 299,792,458 m/s divide, nothing is rounded into the physics. The frequency road agrees: c/λ = 545.077196 THz. A watt of that green is a river of 2.769e+18 photons every second, and a mole of them (Avogadro's own count) banks 217.502847 kJ — chemistry-scale energy, which is why one photon can pay for one chemical step.
Formula
E = h·f = h·c/λ · E(eV) ≈ 1,239.84 / λ(nm) · h = 6.62607015e-34 J·s (exact) · 1 eV = 1.602176634e-19 J (exact)
Planck's relation prices light by the packet: the energy of one photon is Planck's constant times the frequency, and since every electromagnetic wave obeys c = f·λ, wavelength works just as well. The two constants in the numerator are FIXED by the SI — h and c are exact defined values, so the only approximation anywhere on this page is the wavelength you typed. The electron-volt is a bookkeeper's unit: the energy one electron gains crossing one volt, exact at 1.602176634e-19 J.
Worked Example
- Enter the wavelength in nanometres (550 is green).
- Read one photon's energy in joules and electron-volts.
- Read the photon river: how many packets one watt of this light spends per second.
- Check the band card to see which neighbourhood of the spectrum your light lives in.
Defaults: 550 nm → 3.612e-19 J = 2.254258 eV per photon; 2.769e+18 photons per joule-second; 217.502847 kJ per mole of photons.
Strengths & Limits Of This Model
Where this engine is strong
- Exact SI constants throughout
- Per-joule and per-mole counts derived live
Where it stops
- No photoelectric threshold modelling
- No intensity or exposure pricing
Practical Use Cases
Photochemistry
does one photon pay for one bond
LED and laser work
colour versus photon budget
Teaching
the quantum floor of light
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Photon Energy Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why does shorter wavelength mean more energetic light?
E = h·c/λ divides by the wavelength: halve λ and every packet doubles its energy. Ultraviolet burns where visible light tans because each UV packet carries enough energy to break a molecular bond, and red light's packets mostly cannot. The energy is per-packet, not per-watt — a dim blue can do chemistry a bright red cannot.
How many photons are in a watt of light?
Depends on the colour: divide the wattage by one photon's energy. At 550 nm that is 2.769e+18 photons per second per watt. Longer wavelengths carry smaller packets, so the same watt is MORE photons — the river runs deeper but each drop is weaker.
What is an electron-volt, and why do this page's kind of numbers use it?
The energy one electron gains crossing a one-volt gap: 1.602176634e-19 J, exact. Atomic and photon energies live at that scale, so eV keeps the digits readable — 2.254258 eV instead of 0.00000000000000000036 J. The SI fixed the conversion, so nothing here is rounded.
Where do the visible band's edges sit?
Roughly 380 to 750 nm, with the exact borders a physiology question rather than a physics one — eyes vary. This page's band card uses conventional edges and quotes them as conventions: violet near 380, red dying near 750, everything else a neighbour's label away.
What does 1,239.84 over nanometres compute?
A bookkeeping shortcut: h·c expressed in eV·nm is 1,239.841984 (exact constants, one division), so E in eV is that constant over λ in nm. The page derives it from h and c rather than pasting it — the derivation is the check.
Why does one mole of photons matter?
Chemistry counts by moles: a photon's energy times Avogadro's number converts the quantum price to a lab-scale one. At 550 nm that is 217.502847 kJ per mole — squarely in the range of chemical bond energies, which is the whole reason light drives chemistry one packet at a time.
Is a photon's energy shared across the wave?
No — that is THE quantum statement. Brighter light of the same colour means MORE photons, not fatter ones. Turn the wavelength shorter and the packets fatten; turn the brightness up and only their count grows. This page prices the packet; the wattage question is separate arithmetic on top.
How does this pair with the wavelength page?
That page divides velocity by frequency to size a wave (λ = v/f) for sound and radio — the classical wavelength. This page takes the wavelength you already know and prices its quantum. Same Greek letter, two different questions; the frequency card here (c/λ) is the hinge between them.