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Photon Energy Calculator

Physics

The light source

Wavelength, frequency, wavenumber and energy are four names for one quantity, tied together by E = h·ν = h·c / λ. Supply whichever you have; every other panel on this page is derived from the single unrounded energy that comes out.

E = h·c / λ — The everyday case: a laser line, an absorption peak or an LED emission wavelength goes in, and every equivalent descriptor comes out.

Laser lines, X-ray anodes and everyday sources, each loading a complete scenario.
Digits kept after the leading one, 0–10.
Scientific notation such as 5.32e2 is accepted.
Only the wavelength changes in a medium — never the energy.
1 for vacuum. Values below 1 are flagged, not rejected.
Off means you typed the vacuum wavelength.

Photon energy

Visible — greenNon-ionising

approximate colour

Energy in joules
3.7339e-19 J
Energy in electronvolts
2.3305 eV
Frequency
563.52 THz
Vacuum wavelength
532 nm

18797 cm⁻¹

ṽ = 1/λ — proportional to energy

224.86 kJ/mol

53.743 kcal/mol

1.2455e-27 kg·m/s

p = h/λ = E/c

532 nm

λ/n at n = 1

3.5407e+15 rad/s

ω = 2πν, for E = ħω

4.1545e-36 kg

The rest mass of a photon is exactly zero

90.754 × kT

kT = 0.02568 eV at 298 K

Where this photon sits

Electromagnetic spectrum from picometres to a thousand kilometres, with the current photon marked at 532 nm, 2.331 eV, in the visible — green region.GammaX-rayUVVisibleInfraredMicrowaveRadio1 pm1000 km1.24 MeV1.24 feV532 nm · 2.331 eV
Sweeping the slider rewrites the wavelength in nanometres and everything above updates with it.

Beam power, pulses and flux

Optical power is photon energy times photons per second, so N = P/E. Leave a field blank to omit that block entirely.

Blank omits the flux block.
dBm is referenced to 1 mW.
Blank omits the photons-per-pulse block.
Blank omits flux density.

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N = P / E

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Photons per square centimetre per second

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Power spread over the beam area

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N = E_pulse / E_photon

Band gap, work function or bond energy

A photon is absorbed, an electron is ejected or a bond is broken only if one photon carries enough energy. The cut-off wavelength λ_max = hc/E is where that stops being true.

Representative room-temperature literature values.
Blank omits the verdict.
Energy-level diagram with an arrow proportional to the photon energy.Ground stateExcited statehν = 2.331 eV

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Enter a threshold energy

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λ_max = hc / E_threshold

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Positive means the photon clears the gap
Blank omits the E/kT ratio.

The same photon energy in every supported unit, all derived from one joule figure.

UnitValueSymbol
eV — electronvolt2.3305eV
keV — kiloelectronvolt0.0023305keV
MeV — megaelectronvolt2.3305e-6MeV
J — joule3.7339e-19J
erg (CGS)3.7339e-12erg
kJ/mol — per mole of photons224.86kJ/mol
kcal/mol — per mole of photons53.743kcal/mol
Eh — hartree (atomic units)0.085645Eh
Ry — rydberg0.17129Ry

About This Tool

Photon Energy Calculator – Wavelength, Frequency and Energy in One Relation

Light comes in indivisible packets, and the energy of one packet is fixed entirely by its colour. That is the whole content of the Planck–Einstein relation, and it is what this photon energy calculator evaluates in every direction: give it a wavelength, a frequency, a spectroscopic wavenumber or an energy in any of nine units, and it returns all the others, along with the photon momentum, the energy per mole and the photon flux a beam of a given power delivers.

The formula behind photon energy

Three equivalent forms describe the same fact, differing only in which descriptor of the wave you happen to have:

E = h·ν   E = h·c / λ   E = h·c·ṽ

Here h = 6.62607015 × 10⁻³⁴ J·s is the Planck constant and c = 299 792 458 m/s the speed of light, both exact by definition since the 2019 SI redefinition. Take the 532 nm line of a frequency-doubled Nd:YAG laser: E = hc/λ gives 3.733921 × 10⁻¹⁹ J, which is 2.330530 eV, a frequency of 563.52 THz and a wavenumber of 18796.99 cm⁻¹. Because energy and wavelength are inversely proportional, halving the wavelength doubles the energy — the reason ultraviolet burns skin that infrared merely warms.

Electronvolts, wavenumbers and moles

Different fields measure the same quantity in their own units, and the conversions are worth memorising. One electronvolt is 1.602176634 × 10⁻¹⁹ J, equivalent to 96.485 kJ/mol or 23.061 kcal/mol once multiplied by the Avogadro constant. The convenience constant hc = 1239.8420 eV·nm turns a wavelength in nanometres straight into electronvolts by simple division. Infrared spectroscopists prefer the wavenumber ṽ = 1/λ in cm⁻¹ because, unlike wavelength, it is directly proportional to energy — a 1650 cm⁻¹ carbonyl stretch is 0.20457 eV, or 19.738 kJ/mol.

A mole of photons is called an einstein, and it is the figure that lets you compare light with chemistry. Those 532 nm photons carry 224.862 kJ/mol — well short of a C–C bond at roughly 350 kJ/mol, which is why green light will not cleave one no matter how bright the beam.

Photon flux, band gaps and the ionisation threshold

Optical power is photons per second times the energy of each, so N = P/E = Pλ/hc. A 5 mW helium–neon laser at 632.8 nm emits 1.5928 × 10¹⁶ photons a second; focused onto a 1 mm² spot that is 1.5928 × 10¹⁸photons·cm⁻²·s⁻¹ at an irradiance of 0.5 W/cm². The same arithmetic answers detector questions: a photon must exceed a semiconductor's band gap to be absorbed, and the cut-off wavelength is λ_max = hc/E_gap. Silicon's 1.12 eV gap cuts off at 1107 nm, which is exactly why silicon photodiodes are blind to 1550 nm telecom light at 0.7999 eV.

Energy does not change when light enters a medium
In a medium of refractive index n the wavelength shortens to λ/n and the phase velocity drops to c/n, but the frequency is set by the source and cannot change — so E = hν stays exactly the same. A green laser is still green underwater.

Above roughly 10 eV, near 124 nm, a single photon can strip an electron from an atom, and the radiation is called ionising. Below it, extra intensity only adds more weak photons: a 2.45 GHz microwave photon is about 10 µeV, a millionth of a bond energy, however many kilowatts the oven draws. Comparing photon energy with the thermal scale kT makes the same point from the other side — at 298 K, kT is 0.0257 eV, so a green photon is 90.75 × kT, far above the thermal noise floor a detector has to beat.

Frequently Asked Questions

Is the Photon Energy Calculator free?

Yes, Photon Energy Calculator is totally free :)

Can I use the Photon Energy Calculator offline?

Yes, you can install the webapp as PWA.

Is it safe to use Photon Energy Calculator?

Yes, any data related to Photon Energy Calculator only stored in your browser (if storage required). You can simply clear browser cache to clear all the stored data. We do not store any data on server.

How does this photon energy calculator work?

Whatever you type — a wavelength, a frequency, a wavenumber or an energy in any of nine units — is converted to SI first, and a single unrounded photon energy in joules is computed from it with the Planck–Einstein relation E = hν = hc/λ. Every other figure on the page, from electronvolts to photon momentum to kJ per mole, is then derived from that one number, so no two panels can disagree because one of them started from a rounded display value.

Why is 532 nm light 2.3305 eV and not 2.331 eV?

Because the tool uses the exact SI values of h, c and e rather than the three-digit constants most textbooks quote. E = hc/λ at exactly 532 nm gives 3.733921 × 10⁻¹⁹ J, which is 2.330530 eV, 224.862 kJ/mol and 53.743 kcal/mol. Rounded constants easily shift the fourth digit, and a wavelength typo of a tenth of a nanometre shifts it too — 531.89 nm, for instance, would give the 2.3310 eV you sometimes see quoted for a green laser.

Does a photon lose energy when it enters glass or water?

No. In a medium of refractive index n the wavelength shortens to λ/n and the phase velocity drops to c/n, but the frequency is fixed by the source and the energy E = hν goes with it. That is why a green laser is still green underwater. The tool shows the vacuum and in-medium wavelengths side by side precisely because this is the most common student mistake in the subject.

What counts as ionising radiation?

The usual working threshold is about 10 eV per photon, which corresponds to a wavelength near 124 nm in the far ultraviolet. Above it a single photon can strip an electron from an atom outright; below it, no number of photons will do so — they can only heat the material. This is why a 2.45 GHz microwave oven photon, at about 10 microelectronvolts, is roughly a million times too weak to break a chemical bond however much power the magnetron delivers.

What is a mole of photons, and why do chemists use it?

One mole of photons, 6.02214076 × 10²³ of them, is called an einstein. Multiplying the single-photon energy by the Avogadro constant puts light on the same scale as bond dissociation energies and activation barriers, which are always quoted per mole. A 532 nm photon is 224.86 kJ/mol, comfortably below the roughly 350 kJ/mol of a C–C bond — so green light alone will not cleave one, while 300 nm ultraviolet at 399 kJ/mol will.

How accurate are the results?

The arithmetic is exact to double precision and uses the 2019 SI-exact values of h, c, e, N_A and k_B, so the conversions are limited only by the precision of the number you enter. The physical caveats lie elsewhere: refractive index varies with wavelength, so a single n is an approximation across a broad spectrum; the band-gap, work-function and bond-energy presets are representative room-temperature literature values that shift with temperature, doping and chemical environment; and the colour swatch is a perceptual approximation, since saturated spectral colours fall outside the sRGB gamut.