Photon Energy to Wavelength Converter

Q: What is the formula to convert photon energy to wavelength?

Use the Planck-Einstein relation E = hc/lambda, rearranged to lambda = hc/E. With energy in electronvolts and wavelength in nanometers this becomes lambda(nm) = 1239.84 / E(eV), or in reverse E(eV) = 1239.84 / lambda(nm).

Q: What is the 1240 rule for eV to nm conversion?

The 1240 rule is the handy approximation lambda(nm) = 1240 / E(eV), because hc equals about 1239.84 eV times nm. It is accurate to roughly 0.01 percent. This calculator uses the exact CODATA constants instead of the rounded 1240.

Q: How do I convert eV to nm?

Divide 1239.84 by the energy in electronvolts: lambda(nm) = 1239.84 / E(eV). Example: a 2 eV photon has wavelength 1239.84 / 2 = about 619.9 nm, which is orange-red visible light.

Q: How do I convert nm to eV?

Use the reverse relation E(eV) = 1239.84 / lambda(nm). Example: a 532 nm green laser photon carries 1239.84 / 532 = about 2.33 eV of energy.

Q: What is the wavelength of a 2 eV photon?

lambda = 1239.84 / 2 = about 619.92 nm. A 2 eV photon sits in the orange-red part of the visible spectrum.

Q: How do I convert joules to photon wavelength?

Convert joules to electronvolts first using 1 eV = 1.602176634e-19 J, then apply lambda = hc/E. Equivalently, lambda(m) = 1.98644586e-25 / E(J) directly.

Q: How do I calculate photon frequency from energy?

From E = h times nu, frequency nu = E / h. This calculator shows it automatically: a 2 eV photon has frequency about 4.836e14 Hz (483.6 THz).

Q: What is wavenumber and how is it calculated?

Wavenumber is 1/lambda, usually expressed in cm^-1, and is widely used in spectroscopy. For lambda = 500 nm: 1 / (500e-7 cm) = 20,000 cm^-1. Enter any wavelength above to get its wavenumber instantly.

Q: What is the energy of a 532 nm green laser photon?

E = 1239.84 / 532 = about 2.331 eV (3.734e-19 J), with frequency about 5.64e14 Hz and wavenumber about 18,797 cm^-1.

Q: What photon energy corresponds to red light at 650 nm?

E = 1239.84 / 650 = about 1.907 eV. This is typical of a red laser pointer diode.

Free eV to nm calculator: convert photon energy to wavelength — and wavelength back to energy — with the exact formula E = hc/λ. Get frequency, wavenumber, the electromagnetic spectrum band and the visible color in one click. 100% client-side; nothing leaves your browser.

Energy ⇄ Wavelength Calculator

Photon energy

Wavelength

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Energy

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Energy (alt units)

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Wavelength

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Wavelength (alt units)

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Frequency ν

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Wavenumber

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Period T

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Visible color

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γ raysX-rayUVVisibleIRµwaveRadio

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The energy–wavelength formula

A photon's energy E and wavelength λ are linked by the Planck–Einstein relation:

E = h·c / λ ⇔ λ = h·c / E ⇔ ν = E / h = c / λ

With Planck's constant h = 6.62607015×10⁻³⁴ J·s, the speed of light c = 299,792,458 m/s and the elementary charge e = 1.602176634×10⁻¹⁹ J, the product hc = 1239.84 eV·nm exactly. That gives the two workhorse forms:

λ (nm) = 1239.84 / E (eV) · E (eV) = 1239.84 / λ (nm)

This is the origin of the popular 1240 rule — a 0.01% approximation handy for mental math, e.g. 1240/620 nm ≈ 2 eV.

Worked examples

What is the wavelength of a 2 eV photon? λ = 1239.84/2 = 619.92 nm (orange-red light).
What is the energy of a 532 nm green laser photon? E = 1239.84/532 = 2.331 eV.
How many eV is a 650 nm red photon? E = 1239.84/650 = 1.907 eV — typical red laser pointer.
Convert 13.6 eV to nm: λ = 1239.84/13.6 = 91.18 nm (hydrogen ionization / Rydberg photon).
Frequency from eV: a 2 eV photon has ν = E/h = 483.6 THz.
Wavenumber from wavelength: 500 nm → 1/λ = 20,000 cm⁻¹.
kJ/mol to wavelength: 100 kJ/mol → 1.036 eV → ≈1196 nm (near-IR).

Common photon energies and wavelengths

Source	Wavelength	Energy
Red laser pointer	650 nm	—
He–Ne laser	632.8 nm	—
Green laser pointer	532 nm	—
Blue laser / LED	450 nm	—
Violet (Blu-ray) laser	405 nm	—
UV-C germicidal lamp	254 nm	—
Hydrogen Lyman-α	121.6 nm	—
Hydrogen ionization (Rydberg)	91.18 nm	—
Cu Kα X-ray (crystallography)	0.154 nm	—
Mo Kα X-ray	0.0709 nm	—
⁶⁰Co gamma ray	0.000932 nm	—
Thermal IR (room temperature)	10 µm	—
Wi-Fi 2.4 GHz	12.49 cm	—
FM radio 100 MHz	3 m	—

Energies are computed live from the wavelength column using the same exact constants as the converter above.

Electromagnetic spectrum: wavelength vs photon energy

Band	Wavelength range	Photon energy range
Gamma rays	< 0.01 nm	> 124 keV
X-rays	0.01 – 10 nm	124 eV – 124 keV
Ultraviolet	10 – 400 nm	3.1 – 124 eV
Visible light	400 – 750 nm	1.65 – 3.10 eV
Infrared	750 nm – 1 mm	1.24 meV – 1.65 eV
Microwave	1 mm – 1 m	1.24 µeV – 1.24 meV
Radio waves	> 1 m	< 1.24 µeV

Note: E = hc/λ applies to photons only. Massive particles such as electrons use the de Broglie relation λ = h/p instead.

Frequently asked questions

What is the formula to convert photon energy to wavelength?

Use the Planck-Einstein relation E = hc/lambda, rearranged to lambda = hc/E. With energy in electronvolts and wavelength in nanometers this becomes lambda(nm) = 1239.84 / E(eV), or in reverse E(eV) = 1239.84 / lambda(nm).

What is the 1240 rule for eV to nm conversion?

The 1240 rule is the handy approximation lambda(nm) = 1240 / E(eV), because hc equals about 1239.84 eV times nm. It is accurate to roughly 0.01 percent. This calculator uses the exact CODATA constants instead of the rounded 1240.

How do I convert eV to nm?

Divide 1239.84 by the energy in electronvolts: lambda(nm) = 1239.84 / E(eV). Example: a 2 eV photon has wavelength 1239.84 / 2 = about 619.9 nm, which is orange-red visible light.

How do I convert nm to eV?

Use the reverse relation E(eV) = 1239.84 / lambda(nm). Example: a 532 nm green laser photon carries 1239.84 / 532 = about 2.33 eV of energy.

What is the wavelength of a 2 eV photon?

lambda = 1239.84 / 2 = about 619.92 nm. A 2 eV photon sits in the orange-red part of the visible spectrum.

How do I convert joules to photon wavelength?

Convert joules to electronvolts first using 1 eV = 1.602176634e-19 J, then apply lambda = hc/E. Equivalently, lambda(m) = 1.98644586e-25 / E(J) directly.

How do I calculate photon frequency from energy?

From E = h times nu, frequency nu = E / h. This calculator shows it automatically: a 2 eV photon has frequency about 4.836e14 Hz (483.6 THz).

What is wavenumber and how is it calculated?

Wavenumber is 1/lambda, usually expressed in cm^-1, and is widely used in spectroscopy. For lambda = 500 nm: 1 / (500e-7 cm) = 20,000 cm^-1. Enter any wavelength above to get its wavenumber instantly.

What is the energy of a 532 nm green laser photon?

E = 1239.84 / 532 = about 2.331 eV (3.734e-19 J), with frequency about 5.64e14 Hz and wavenumber about 18,797 cm^-1.

What photon energy corresponds to red light at 650 nm?

E = 1239.84 / 650 = about 1.907 eV. This is typical of a red laser pointer diode.

What is the energy of a 254 nm UV-C photon?

E = 1239.84 / 254 = about 4.881 eV. UV-C photons (100 to 280 nm) are energetic enough to damage DNA, which is why 254 nm mercury lamps are used for germicidal sterilization.

What is the energy of a Cu K-alpha X-ray photon at 0.154 nm?

E = 1239.84 / 0.154 = about 8050.9 eV, or 8.05 keV, in the X-ray band. Copper K-alpha radiation is the standard wavelength used in X-ray crystallography.

What is the wavelength of a 13.6 eV photon?

lambda = 1239.84 / 13.6 = about 91.18 nm, in the extreme ultraviolet. 13.6 eV is the ionization energy of hydrogen, also called the Rydberg energy.

How do I convert kJ/mol to photon wavelength?

1 eV per photon equals 96.485 kJ/mol. Divide kJ/mol by 96.485 to get eV, then lambda(nm) = 1239.84 / E(eV). Example: 100 kJ/mol becomes 1.036 eV, giving about 1196 nm in the near infrared.

Does E = hc/lambda work for electrons or only photons?

Only for photons, which are massless. For electrons use the de Broglie relation lambda = h / p, which also needs the electron mass and kinetic energy - a different formula.

Why do astronomers and chemists use angstrom, cm^-1 and GHz instead of nm and eV?

Convenience of scale: X-ray astronomers use angstroms (1 A = 0.1 nm) and keV, chemists use wavenumbers (cm^-1) for vibrational spectra, and radio astronomers use GHz or MHz for frequency. This converter outputs all of these at once.

Free tool by NRTechStudio

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