Photon Energy & Wavelength Calculator
Convert light between wavelength, frequency and energy. Enter any one of them, or a wavenumber, and see the photon energy in eV, J or kJ/mol, the band of the spectrum and its colour, plus the photoelectric effect and de Broglie wavelengths.
How to Use
- Pick what you know: a wavelength, a frequency, a photon energy or a wavenumber. The other three fields fill in as answers.
- Choose a unit beside each field: nm, µm or m for wavelength; Hz up to EHz for frequency; eV, J or kJ/mol for energy; cm⁻¹ for wavenumber.
- Read the band of the spectrum, from radio to gamma rays, with the colour shown when the light is visible.
- For the photoelectric effect, enter the light’s wavelength and pick a metal (or type a work function) to get the fastest electrons; or enter a measured stopping voltage to find the work function.
- For matter waves, pick an electron, proton, neutron or any mass, and give its speed, kinetic energy or accelerating voltage.
Worked Example
A green laser. λ = 532 nm = 5.32 × 10⁻⁷ m. f = c ÷ λ = 299,792,458 ÷ 5.32 × 10⁻⁷ = 5.635 × 10¹⁴ Hz (563.5 THz), and E = hf = 6.62607015 × 10⁻³⁴ × 5.635 × 10¹⁴ = 3.734 × 10⁻¹⁹ J. Dividing by 1.602176634 × 10⁻¹⁹ J/eV gives 2.331 eV, or 224.9 kJ per mole of photons.
The photoelectric effect. Mercury-lamp UV at 253.7 nm has E = 1,239.84 ÷ 253.7 = 4.887 eV. On zinc, with a typical work function of 4.33 eV, the fastest electrons leave with KE = 4.887 − 4.33 = 0.557 eV, so a stopping voltage of 0.557 V halts them. The threshold is 1,239.84 ÷ 4.33 = 286.3 nm.
The common mistake: leaving the wavelength in nanometres. Putting λ = 532 straight into E = hc ÷ λ gives 3.734 × 10⁻²⁸ J, or 2.331 × 10⁻⁹ eV, a billion times too small. Either convert to metres first (5.32 × 10⁻⁷ m) or use the shortcut E (eV) = 1,239.84 ÷ λ (nm), which gives the right 2.331 eV.
Show Work
Formulas
From Planck’s Quanta to Matter Waves
In December 1900 Max Planck explained the spectrum of glowing bodies by assuming that energy is exchanged in lumps of E = hf. He thought of it as a mathematical trick. In 1905 Albert Einstein took it literally: light itself comes in quanta, which explained why Heinrich Hertz (1887) and Wilhelm Hallwachs (1888) found that ultraviolet light, but not visible light, discharges a zinc plate. Robert Millikan set out to disprove Einstein’s equation and instead confirmed it in 1916, measuring h on the way. Einstein’s Nobel Prize for 1921 was awarded for the photoelectric law, not relativity.
In his 1924 doctoral thesis Louis de Broglie turned the idea around: if waves can behave like particles, particles should behave like waves, with λ = h ÷ p. In 1927 Clinton Davisson and Lester Germer, firing 54 eV electrons at a nickel crystal, and George Paget Thomson, sending electrons through thin films, both saw electron diffraction. De Broglie received the Nobel Prize in 1929; Davisson and Thomson shared it in 1937.
Since the 2019 redefinition of the SI, h, c, e and the Avogadro constant all have exact values, so every conversion on this page is exact apart from rounding. Particle masses are the NIST CODATA 2022 values. Band edges are conventions (the visible range is given as 380–750 nm here; some sources use 400–700 nm), and colours are an approximate sRGB rendering from the CIE 1931 colour-matching functions.
About This Tool
This calculator links the four ways of describing a photon: wavelength, frequency, energy and wavenumber. Enter any one and the other three appear in their own fields, in the unit you choose, along with the energy per mole, the photon’s momentum and where it sits in the spectrum. The photoelectric mode splits a photon’s energy into the work function and the electron’s kinetic energy (the work functions offered are typical values for clean polycrystalline metals; real surfaces differ by a few tenths of an eV), and the de Broglie mode gives the wavelength of a moving particle, always using the relativistic momentum and showing how far off the simple formula would be.
Everything runs in your browser; nothing you enter is sent anywhere.
Related tools: Frequency & Wavelength Converter, Black-Body Radiation Calculator, and Special Relativity Calculator.
Frequently Asked Questions
How do I convert a wavelength to photon energy?
Use E = hc ÷ λ. In handy units, E in eV = 1,239.84 ÷ λ in nm. A 532 nm green laser photon has 1,239.84 ÷ 532 = 2.331 eV (3.734 × 10⁻¹⁹ J); red light at 700 nm has 1.771 eV and violet at 400 nm has 3.100 eV.
What is one electronvolt in wavelength, frequency and kJ/mol?
A 1 eV photon has a wavelength of 1,239.84 nm (near infrared), a frequency of 241.8 THz and a wavenumber of 8,065.5 cm⁻¹. A mole of such photons carries 96.49 kJ, which is how chemists compare light with bond energies.
Why does UV light knock electrons out of zinc but red light does not?
Each photon acts alone. Zinc’s work function is typically 4.33 eV, so only light shorter than 286.3 nm can knock out an electron. A 700 nm photon has just 1.771 eV, however bright the lamp, while 253.7 nm UV from a mercury lamp has 4.887 eV and releases electrons with up to 0.557 eV to spare.
How much energy does a microwave oven photon have?
Microwave ovens run at 2.45 GHz, a wavelength of 12.24 cm. Each photon has only 1.013 × 10⁻⁵ eV, about 230,000 times less than a green photon; ovens heat food by sheer numbers of photons making water molecules rotate, not by breaking bonds.
What is the de Broglie wavelength of an electron?
λ = h ÷ p. An electron accelerated through 54 V (as in the 1927 Davisson–Germer experiment) has a momentum of 3.970 × 10⁻²⁴ kg·m/s and a wavelength of 0.1669 nm, about the spacing of atoms in a crystal, which is why crystals diffract electrons. Through 100 V it is 0.1226 nm.
How do I use the Photon Energy & Wavelength Calculator?
Just type your numbers. The answer shows up right away — there is no button to press. Change anything and it updates by itself.
Do I need to install or sign up for anything?
Not at all — it runs in the browser with nothing to install and no account. After it loads once, it even works without an internet connection.
Is my information private?
Yes. Everything happens in your browser. Nothing you type is sent to a server or saved anywhere.
Common Use Cases
Lasers and LEDs
A 532 nm laser photon carries 2.331 eV; a 1 mW beam of them is 2.678 × 10¹⁵ photons every second.
Spectroscopy
Turn the CO₂ infrared band at 2,349 cm⁻¹ into 4.257 µm, 70.42 THz and 0.2912 eV.
Radio and microwaves
An FM station on 100 MHz has a 2.998 m wavelength, so a quarter-wave antenna is about 75 cm long.
Radiation physics
The 662 keV gamma ray of caesium-137 has a wavelength of 1.873 pm and a frequency of 1.601 × 10²⁰ Hz.
Electron microscopes
A 1 MeV electron has a de Broglie wavelength of 0.8719 pm; the non-relativistic formula would say 1.226 pm, 40.7% too long.
Neutron scattering
A thermal neutron at 2,200 m/s (25.3 meV) has a wavelength of 0.1798 nm, matched to the spacing of atoms.
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