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Photon Energy Calculator

Find a photon's energy in eV and joules from its wavelength.

How it is calculated

A photon's energy is inversely proportional to its wavelength: E = h·c ÷ λ and its frequency is f = c ÷ λ. The wavelength λ is entered in nanometres and converted to metres, h = 6.62607015 × 10⁻³⁴ J·s is Planck's constant and c = 299,792,458 m/s is the speed of light in vacuum. The energy is shown in joules and in electronvolts (E in joules ÷ 1.602176634 × 10⁻¹⁹), the frequency in hertz. Joules and hertz are written in scientific notation with three decimals. Wavelength must be positive. Shorter wavelengths, such as blue or ultraviolet light, carry more energy per photon than longer, red ones.

Worked example

Green light with λ = 500 nm = 5 × 10⁻⁷ m. Energy: 6.626 × 10⁻³⁴ × 2.998 × 10⁸ ÷ 5 × 10⁻⁷ = 3.973 × 10⁻¹⁹ J. Converted to electronvolts: 3.973 × 10⁻¹⁹ ÷ 1.602 × 10⁻¹⁹ = 2.480 eV. Frequency: 2.998 × 10⁸ ÷ 5 × 10⁻⁷ = 5.996 × 10¹⁴ Hz.

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Common mistakes

  • Entering wavelength in metres or micrometres. The field is in nanometres, so 500 nm, not 0.0000005.
  • Entering frequency or wavenumber in the wavelength field.
  • Assuming a wavelength in air or glass. The calculation uses the vacuum speed of light.
  • Expecting brighter light to give a larger photon energy. Brightness changes the number of photons, not the energy of each one.

About this calculator

This calculator finds a photon's energy from its wavelength using E = hc/λ, where h is Planck's constant, c is the speed of light, and λ is the wavelength; it also reports the corresponding frequency (f = c/λ). Shorter wavelengths (like UV or X-rays) carry more energy per photon than longer ones (like infrared or radio), since energy is inversely proportional to wavelength.

Physics and chemistry students, and researchers in spectroscopy or photochemistry, use it to convert between a spectral line's wavelength and its energy in electronvolts (eV) or joules — useful for identifying whether a photon can ionize an atom, drive a photochemical reaction, or match an electronic transition.

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Comments (1)

  • Editorial team

    A handy shortcut for checking the output: the energy in electronvolts is about 1239.84 divided by the wavelength in nanometres. At 620 nm that gives about 2.00 eV. The same idea tells you whether light can excite a material, since a band gap of 1.12 eV, as in silicon, corresponds to roughly 1107 nm. The wavelength is taken as in vacuum. Which wavelengths are you most often converting?

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