Wavelength to Energy Calculator
Enter a photon wavelength and choose your preferred energy unit to get the photon energy instantly using the Planck-Einstein relation. Switch the solve mode to start from energy or frequency instead. Results update as you type across six energy units (eV, meV, keV, MeV, J, kJ/mol) and six wavelength units (nm, angstrom, pm, um, mm, m).
Formula
Worked example
Green light at 550 nm: E = (6.626 x 10^-34 J·s x 2.998 x 10^8 m/s) / (550 x 10^-9 m) = 3.613 x 10^-19 J = 2.254 eV. Per mole: 3.613 x 10^-19 J x 6.022 x 10^23 / 1000 = 217.6 kJ/mol.
The Planck-Einstein relation and how this calculator uses it
Every photon carries a fixed quantum of energy that depends only on its frequency. The relationship is E = hf, where h is Planck's constant (6.626 x 10^-34 J·s) and f is frequency. Because frequency and wavelength are linked by the speed of light (f = c / lambda), the same equation becomes E = hc / lambda. This calculator accepts wavelength, energy, or frequency as the starting point and derives all the others. You can work in any of six energy units (eV, meV, keV, MeV, J, kJ/mol) and six wavelength units (nm, angstrom, pm, um, mm, m).
Energy units for different parts of the spectrum
Electronvolts (eV) are the standard unit in atomic, molecular and optical physics because photon energies in the ultraviolet and visible range conveniently sit between about 1.8 eV and 10 eV. For softer radiation such as far infrared and microwave, milli-electronvolts (meV) or micro-electronvolts (ueV) are more practical. For hard X-rays and gamma rays, keV and MeV are used. In chemistry and photochemistry, kJ/mol is preferred because it directly compares photon energy to chemical bond energies: C-C bonds require about 347 kJ/mol to break, so only UV photons (above about 350 kJ/mol, or roughly 360 nm) carry enough energy per photon to do so.
Visible-light colour and energy
Violet light at 380 nm has a photon energy of about 3.26 eV, and deep red at 700 nm has about 1.77 eV. The human eye perceives this as a spectrum from violet through blue, green, yellow, and orange to red. Solar panels convert photons above the bandgap energy of silicon (about 1.1 eV, or roughly 1100 nm) into electron-hole pairs, which is why they respond well to the entire visible range and also to near-infrared light.
Frequency, wavelength, and why photon energy is always positive
Frequency and wavelength are inversely proportional at the speed of light. A 550 nm green photon has a frequency of about 545 THz (5.45 x 10^14 Hz). Because photon energy is proportional to frequency and all electromagnetic radiation travels at c in a vacuum, shorter wavelengths always carry more energy per photon. This is why UV and X-ray radiation are ionising (enough energy to eject electrons from atoms) while radio waves are not.
Electromagnetic spectrum: wavelength and photon energy
| Region | Wavelength range | Energy per photon (eV) | Typical source |
|---|---|---|---|
| Gamma ray | < 0.01 nm | > 124 keV | Nuclear reactions, cosmic events |
| X-ray | 0.01 nm - 10 nm | 124 eV - 124 keV | Medical imaging, crystallography |
| Extreme UV | 10 nm - 120 nm | 10 eV - 124 eV | Solar corona, EUV lithography |
| Near UV | 120 nm - 400 nm | 3.1 eV - 10 eV | Sunlight, blacklights |
| Violet | 380 nm - 450 nm | 2.75 eV - 3.26 eV | Blue LEDs |
| Blue | 450 nm - 495 nm | 2.51 eV - 2.75 eV | Sky, ocean |
| Green | 495 nm - 570 nm | 2.18 eV - 2.51 eV | Leaves, lasers |
| Yellow/Orange | 570 nm - 620 nm | 2.00 eV - 2.18 eV | Sodium lamps |
| Red | 620 nm - 700 nm | 1.77 eV - 2.00 eV | Red LEDs, blood |
| Near IR | 700 nm - 1000 um | 1.24 meV - 1.77 eV | Remote controls, heat |
| Mid IR | 1000 um - 100 mm | 12.4 ueV - 1.24 meV | Thermal imaging |
| Microwave | 1 mm - 30 cm | 4 ueV - 1.24 meV | Radar, Wi-Fi, ovens |
| Radio wave | > 30 cm | < 4 ueV | AM/FM broadcasts, NMR |
Approximate energy ranges per photon for each region. Visible light is highlighted for reference.
Frequently asked questions
What formula does the wavelength to energy calculator use?
It uses the Planck-Einstein relation E = hc / lambda, where h is Planck's constant (6.626 x 10^-34 J·s), c is the speed of light in a vacuum (2.998 x 10^8 m/s), and lambda is the wavelength in metres. In practice the product hc equals about 1239.84 eV·nm, so a quick mental calculation for visible light is to divide 1240 by the wavelength in nm to get energy in eV.
How do I convert wavelength in nm to energy in eV?
Divide 1240 by the wavelength in nanometres. For example, 620 nm (red light) gives roughly 1240 / 620 = 2.0 eV. The exact constant is 1239.84 eV·nm. This shortcut works for any wavelength: 248 nm (UV excimer laser) gives about 5.0 eV, and 1550 nm (fibre-optic telecom) gives about 0.80 eV.
What is an electronvolt (eV)?
An electronvolt is the kinetic energy gained by a single electron when accelerated through a potential difference of one volt. It equals 1.602 x 10^-19 joules. Because atomic-scale energies are so small in SI units, eV is the natural unit for photon energies, ionisation energies, and semiconductor bandgaps.
Why is photon energy in kJ/mol useful?
Chemists express bond energies in kJ/mol, so comparing photon energies in the same unit shows at a glance whether a photon can break a particular bond. For example, the O-H bond takes about 460 kJ/mol to break homolytically. A UV photon at 260 nm has an energy of about 460 kJ/mol, which is why UV is more damaging to molecules than visible light.
Can I calculate the wavelength from energy or frequency?
Yes. Switch the "Solve for" dropdown to "Wavelength from energy" or "Energy from frequency". In wavelength-from-energy mode, enter an energy value and choose its unit; the calculator solves lambda = hc / E and shows the wavelength in nm together with the frequency and spectrum region. In frequency mode, enter a frequency in THz, GHz, MHz, kHz, or Hz.
Which electromagnetic spectrum region does a wavelength fall in?
The calculator labels each result with the spectrum region: gamma ray (below 0.01 nm), X-ray (0.01 to 10 nm), ultraviolet (10 to 400 nm), visible light (400 to 700 nm), infrared (700 nm to 1 mm), microwave (1 mm to 30 cm), and radio wave (above 30 cm). These boundaries are approximate, since different fields use slightly different conventions.