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💡 Quick rule: E (eV) ≈ 1240 / λ (nm). Green 550 nm → 2.25 eV. UV 300 nm → 4.13 eV.
Photon Energy
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Sources & Methodology
Planck constant and speed of light from NIST CODATA 2018 exact values. Electronvolt definition from BIPM SI.
NIST CODATA 2018 — Planck Constant
Exact defined value h = 6.62607015×10⁻³⁴ J·s (exact since 2019 SI redefinition), used for all photon energy calculations E = hc/λ in this calculator.
BIPM SI Units — Electronvolt Definition
BIPM definition: 1 eV = 1.602176634×10⁻¹⁹ J (exact by definition since 2019 SI redefinition), used for all joules-to-eV conversions in this calculator.
Methodology: E (J) = h×c/λ. E (eV) = E(J) / 1.602176634×10⁻¹⁹. Convenient: E (eV) = 1239.84 eV·nm / λ(nm). Frequency f = c/λ. Wavenumber k = 2π/λ (rad/m). Spectroscopic wavenumber = 1/λ(cm) (cm⁻¹). Molar energy = E(J) × 6.02214076×10²³ / 1000 (kJ/mol). Constants used: h = 6.62607015×10⁻³⁴ J·s, c = 299792458 m/s, 1 eV = 1.602176634×10⁻¹⁹ J, N_A = 6.02214076×10²³.
⏱ Last reviewed: April 2026
How to Calculate Photon Energy from Wavelength
Photon energy is the energy carried by a single quantum of electromagnetic radiation. It depends entirely on frequency (and therefore wavelength): shorter wavelength means higher frequency means higher energy. This is why UV light causes sunburn and X-rays penetrate tissue, while radio waves pass through the body harmlessly.
The Planck-Einstein Formula
E = hc / λ E (eV) ≈ 1240 / λ (nm)
h = 6.626×10⁻³⁴ J·s (Planck constant) • c = 2.998×10⁸ m/s • λ = wavelength (m)
Green light 550 nm: E = 1240/550 = 2.25 eV = 3.61×10⁻¹⁹ J
UV-B 300 nm: E = 1240/300 = 4.13 eV = 6.62×10⁻¹⁹ J
X-ray 0.1 nm: E = 1240/0.1 = 12,400 eV = 12.4 keV
Green light 550 nm: E = 1240/550 = 2.25 eV = 3.61×10⁻¹⁹ J
UV-B 300 nm: E = 1240/300 = 4.13 eV = 6.62×10⁻¹⁹ J
X-ray 0.1 nm: E = 1240/0.1 = 12,400 eV = 12.4 keV
Photon Energy by Spectrum Region
| Spectrum Region | Wavelength | Energy (eV) | Effect |
|---|---|---|---|
| FM radio | 3 m | 4.1×10⁻⁷ eV | Non-ionising, penetrates walls |
| Microwave (WiFi) | 12.5 cm | 9.9×10⁻⁶ eV | Non-ionising, heats water molecules |
| IR (thermal) | 10 µm | 0.124 eV | Felt as heat, absorbed by skin |
| Red light | 700 nm | 1.77 eV | Visible, used in laser therapy |
| Green light | 550 nm | 2.25 eV | Peak human eye sensitivity |
| Violet light | 380 nm | 3.26 eV | Visible, highest visible energy |
| UV-A | 350 nm | 3.54 eV | Tanning, mild DNA damage |
| UV-C | 254 nm | 4.88 eV | Germicidal, breaks DNA bonds |
| Soft X-ray | 10 nm | 124 eV | Ionising, medical imaging |
| Hard X-ray | 0.1 nm | 12,400 eV | Deep tissue penetration |
💡 Ionisation threshold: Radiation becomes ionising when photon energy exceeds about 10 eV (λ < 124 nm), enough to eject electrons from atoms. UV-C at 4.9 eV is below the ionisation threshold but can break DNA’s hydrogen bonds (about 3–4 eV). X-rays at 12+ keV easily ionise tissue atoms. Radio waves and microwave photons (<0.001 eV) cannot cause ionisation regardless of intensity.
Frequently Asked Questions
E = hf = hc/λ. Using h = 6.626×10⁻³⁴ J·s and c = 2.998×10⁸ m/s. Convenient: E(eV) = 1240/λ(nm). For green 550 nm: E = 1240/550 = 2.25 eV = 3.61×10⁻¹⁹ J. Higher frequency (shorter wavelength) = higher energy per photon.
1 eV = 1.602176634×10⁻¹⁹ J (exact). It is the energy gained by one electron accelerating through 1 volt of potential. Photon energy ranges: radio ~10⁻⁷ eV, visible light 1.8–3.3 eV, UV 3–124 eV, X-rays 100 eV–100 keV, gamma rays >100 keV. The eV is convenient because atomic bond energies are typically 1–10 eV.
Visible light (380–700 nm): 1.77 eV (red, 700 nm) to 3.26 eV (violet, 380 nm). Green at 550 nm: 2.25 eV. A 1 mW green laser emits 2.25×10¹⁵ photons/second. One mole of 550 nm photons carries 217 kJ of energy (one einstein). This is enough to break weak chemical bonds and drive photochemical reactions like photosynthesis.
UV-A (315–400 nm): 3.10–3.94 eV. UV-B (280–315 nm): 3.94–4.43 eV. UV-C (100–280 nm): 4.43–12.4 eV. UV-C at 254 nm (4.88 eV) is used for germicidal sterilisation because its energy can break pyrimidine dimers in DNA (requiring ~4–5 eV). Ozone absorbs UV-C and most UV-B, protecting Earth’s surface.
Soft X-rays (10–0.1 nm): 124 eV to 12.4 keV. Hard X-rays (<0.1 nm): >12.4 keV. Medical chest X-ray: 20–150 keV photons. Quick formula: E(eV) = 1240/λ(nm). For 0.01 nm: E = 1240/0.01 = 124,000 eV = 124 keV. These energies easily exceed atomic ionisation potentials (hydrogen: 13.6 eV), making X-rays strongly ionising.
Inversely proportional: E = hc/λ. Halving wavelength doubles energy. Radio at 1 m: ~1.2×10⁻⁶ eV. Visible green at 550 nm: 2.25 eV. UV at 100 nm: 12.4 eV. X-ray at 0.1 nm: 12,400 eV. Gamma at 1 pm: 1.24 MeV. This 13-order-of-magnitude energy span explains why different EM radiation has radically different biological effects.
λ(nm) = 1240/E(eV). For 2.0 eV: λ = 620 nm (red). For 3.0 eV: 413 nm (violet). For 10 eV: 124 nm (UV). For 10 keV: 0.124 nm (X-ray). The constant 1240 eV·nm = h×c converted to eV·nm units. Exact value: h×c = 6.626×10⁻³⁴ × 2.998×10⁸ = 1.986×10⁻²⁵ J·m = 1239.84 eV·nm.
h = 6.62607015×10⁻³⁴ J·s (exact by 2019 SI redefinition). It relates photon energy to frequency: E = hf. Reduced form: ℏ = h/(2π) = 1.0546×10⁻³⁴ J·s. Planck constant is fundamental to all of quantum mechanics, setting the scale of quantisation in atomic energy levels, the uncertainty principle, and blackbody radiation.
FM radio at 100 MHz: E = hf = 6.626×10⁻³⁴ × 10⁸ = 6.6×10⁻²⁶ J = 4.1×10⁻⁷ eV per photon. This is far too small to ionise atoms or break chemical bonds (which require ≥1 eV). Radio waves are non-ionising; they heat tissue only through bulk dielectric losses. Their safety limit is determined by heating power density, not individual photon energy.
One einstein = energy of one mole of photons = E(J) × 6.022×10²³. For 550 nm green: 3.61×10⁻¹⁹ J × 6.022×10²³ = 217 kJ/mol. This is used in photochemistry to compare light energy to bond dissociation energies (C-H bond: 413 kJ/mol). Photosynthesis requires about 8 einsteins per mole of CO₂ reduced. UV-B at 300 nm: 399 kJ/mol, close to the energy of carbon-carbon double bonds (614 kJ/mol).
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