Photon Energy Calculator
Convert light wavelength in nanometers into single-photon energy in joules and electronvolts.
How the result changes
Only wavelength changes; all other inputs stay at your current values. The line illustrates the formula across the shown range. This is not a forecast of real-world outcomes.
About this calculation
Photon Energy Calculator: Convert light wavelength in nanometers into single-photon energy in joules and electronvolts. Enter wavelength; the reported quantities are photon energy (J), photon energy (eV). The equations and examples below show how those inputs lead to the result and where the model's assumptions matter.
Formula
E = h c / wavelength
Worked example
With Wavelength: 500 nm, photon energy is 0 J.
Method and inputs
Use the Planck relation E = hc/lambda, where h is the defined Planck constant and c is the defined vacuum speed of light. Convert nanometers to meters before dividing, then convert joules to electronvolts with the exact joule-per-electronvolt factor.
Worked example
At 500 nanometers, the energy is about 3.973 × 10^-19 joule for one photon, or approximately 2.48 electronvolts. Halving the wavelength to 250 nanometers doubles both energy readings. This inverse relation is an effective mental check.
Wavelength and energy move in opposite directions. At 1,000 nanometers a photon has roughly half the energy of one at 500 nanometers, about 1.24 electronvolts. This comparison is useful when checking infrared against visible-light results, but it does not tell you how many photons a detector receives.
How to read the result
Photon energy describes one quantum, not the total power of a light source. A bright lamp can deliver many photons at the same individual energy. Multiplying by the number of photons would give a total radiant energy, which this single-photon form does not calculate.
If the source provides frequency instead of wavelength, calculate h times frequency directly rather than entering a frequency into the nanometer box. A medium can shorten wavelength without changing frequency, so the vacuum wavelength convention matters. For many-photon beams, multiply single-photon energy by photon count to estimate beam energy.
Assumptions and checks
The formula assumes electromagnetic radiation in vacuum when relating frequency to wavelength; refraction changes wavelength inside a material but not the photon frequency at the boundary. The input is vacuum wavelength, and relativistic or material interaction effects are outside this calculation.
Do not enter a wavelength in micrometers as though it were nanometers: one micrometer equals one thousand nanometers. Also avoid confusing electronvolts with volts; an electronvolt is an energy unit. The joule result is extremely small, so scientific notation is expected.
Common questions
What does the photon energy calculator calculate?
It multiplies Planck’s constant by light frequency inferred from wavelength and shows the result in joules and electronvolts. The value belongs to a single photon at the specified vacuum wavelength.
When should I recalculate or check another method?
Recalculate when the source wavelength changes or when a spectral line is specified in frequency instead. Check a spectroscopy reference if energy levels, bandwidth or absorption in a medium are the actual quantities of interest.
Sources and references
Inputs
- Wavelength (nm)