Photon Energy Calculator (E = hf = hc/λ) in eV and Joules
By Santaji GadepatilinUpdated October 30, 2026
A photon's energy depends only on its frequency, E = hf, or equivalently on its wavelength, E = hc/λ. This photon energy calculator converts a wavelength into energy in eV and joules, frequency, energy per mole and the number of photons a light source emits per second — useful for lasers, LEDs, photodiodes and phototransistors and photochemistry.
How to use the photon energy calculator
Enter the wavelength in nanometres. The calculator returns the frequency in THz, the photon energy in electronvolts and joules and the energy of one mole of photons. Enter the optical power of a source to find how many photons it emits each second. To go from energy to wavelength, clear the wavelength and enter the energy in eV.
Photon energy formula
E = h f = h c ÷ λ
E (eV) ≈ 1239.84 ÷ λ (nm) · N = P ÷ E
h = 6.62607015 × 10⁻³⁴ J·s (Planck constant), c = 299,792,458 m/s and 1 eV = 1.602176634 × 10⁻¹⁹ J — all exact SI values.
Example
A 532 nm green laser with 1 mW output:
- f = 299,792,458 ÷ 532 × 10⁻⁹ = 563.52 THz
- E = 6.626 × 10⁻³⁴ × 563.52 × 10¹² = 3.7339 × 10⁻¹⁹ J = 2.3305 eV
- Per mole: 224.9 kJ/mol
- Photons per second = 0.001 ÷ 3.7339 × 10⁻¹⁹ = 2.678 × 10¹⁵
Photon energy of common light sources
| Source | Wavelength | Frequency | Energy | Per mole |
|---|---|---|---|---|
| UV-C germicidal lamp | 254 nm | 1180.3 THz | 4.881 eV | 471.0 kJ/mol |
| UV-A / violet laser | 405 nm | 740.2 THz | 3.061 eV | 295.4 kJ/mol |
| Blue LED | 450 nm | 666.2 THz | 2.755 eV | 265.8 kJ/mol |
| Green laser | 532 nm | 563.5 THz | 2.331 eV | 224.9 kJ/mol |
| Red laser | 650 nm | 461.2 THz | 1.907 eV | 184.0 kJ/mol |
| Near-IR (sensors, remotes) | 850 nm | 352.7 THz | 1.459 eV | 140.7 kJ/mol |
| Near-IR LED | 940 nm | 318.9 THz | 1.319 eV | 127.3 kJ/mol |
| Telecom fibre | 1550 nm | 193.4 THz | 0.800 eV | 77.2 kJ/mol |
Notes
- Shorter wavelength means higher energy: UV-C photons (about 4.9 eV) can break chemical bonds, which is why they disinfect.
- An LED’s forward voltage is roughly its photon energy in eV — a blue LED needs about 2.8–3.3 V, a red one about 1.8–2.2 V.
- For a matter wave instead of light, use the de Broglie wavelength calculator.
Frequently asked questions
How do I convert wavelength to photon energy in eV?
E (eV) = 1239.84 ÷ λ (nm). A 532 nm green laser photon carries about 2.33 eV.
What is the energy of a photon in joules?
E = h f = h c ÷ λ, with h = 6.626 × 10⁻³⁴ J·s and c = 2.998 × 10⁸ m/s. A 532 nm photon has about 3.73 × 10⁻¹⁹ J.
How many photons per second does a 1 mW laser emit?
Divide the power by the photon energy. A 1 mW green laser at 532 nm emits about 2.7 × 10¹⁵ photons per second.
Why can't infrared light trigger some photodetectors?
A photon can only free an electron if its energy exceeds the material's band gap. Silicon (1.12 eV) responds up to about 1,100 nm; beyond that, detectors such as InGaAs are needed.