About the Photon Energy Calculator
This photon energy calculator applies the Planck–Einstein relation to find the energy carried by a single photon of light or any other electromagnetic radiation. Enter either the wavelength (in nanometres, micrometres, ångströms and more) or the frequency (Hz to PHz) and it returns the energy in electronvolts and joules, the matching frequency or wavelength, the wavenumber, the energy per mole of photons and the part of the spectrum it belongs to.
It is built for chemistry and physics students working photoelectric-effect and spectroscopy problems, for laser and LED users who want to know photon flux, and for anyone comparing why UV light can break chemical bonds while radio waves cannot. Add a beam power and it tells you how many photons per second the source emits.
The calculation uses the exact SI 2019 values of the Planck constant, the speed of light and the elementary charge, and assumes the wavelength is measured in vacuum (wavelengths in air are about 0.03% shorter, which is negligible for most purposes).
With the default inputs, the photon energy is 2.3305 eV. Change any value above to recalculate instantly.
How to use the photon energy calculator
- 1Choose whether you know the wavelength or the frequency.
- 2Enter the value and pick its unit.
- 3Optionally enter the beam power to get photons per second.
- 4Read the photon energy in eV and joules, plus the converted frequency or wavelength.
Formula and method
Light is absorbed and emitted in packets called photons. Planck and Einstein showed that each photon's energy is proportional to its frequency, E = hf, where h = 6.62607015 × 10⁻³⁴ J·s. Because frequency and wavelength are linked by c = fλ, the energy can also be written E = hc/λ: shorter wavelengths carry more energy.
Dividing joules by the elementary charge (1.602176634 × 10⁻¹⁹ C) gives electronvolts, which leads to the handy shortcut E(eV) ≈ 1239.84/λ(nm). Multiplying by Avogadro's number gives energy per mole of photons for chemistry, and dividing a beam's power by the photon energy gives the number of photons emitted each second.
- E
- Energy of one photon (J or eV)
- h
- Planck constant, 6.62607015 × 10⁻³⁴ J·s
- f
- Frequency (Hz)
- c
- Speed of light in vacuum, 299,792,458 m/s
- λ
- Wavelength in vacuum (m)
Worked examples
532 nm green laser pointer, 5 mW
E = hc/λ = (6.626 × 10⁻³⁴ × 2.998 × 10⁸) ÷ 532 × 10⁻⁹ = 3.734 × 10⁻¹⁹ J = 2.33 eV. A 5 mW beam therefore emits 0.005 ÷ 3.734 × 10⁻¹⁹ ≈ 1.34 × 10¹⁶ photons every second.
Hydrogen Lyman-alpha line, 121.567 nm
1239.84 ÷ 121.567 = 10.20 eV – exactly the energy gap between hydrogen's first two energy levels (13.6 eV × 3/4).
1,550 nm telecom laser
1.55 µm = 1,550 nm, so E = 1239.84 ÷ 1550 = 0.80 eV and the wavenumber is 1 ÷ 1.55 × 10⁻⁴ cm = 6,452 cm⁻¹.
Microwave oven at 2.45 GHz
E = hf = 6.626 × 10⁻³⁴ × 2.45 × 10⁹ = 1.62 × 10⁻²⁴ J = 1.01 × 10⁻⁵ eV, with a wavelength of 12.24 cm. That is far too little energy to break bonds – microwaves heat food by making water molecules rotate.
Frequently asked questions
How do you calculate the energy of a photon?+
Use E = hf if you know the frequency, or E = hc/λ if you know the wavelength, with h = 6.626 × 10⁻³⁴ J·s and c = 2.998 × 10⁸ m/s. For quick work, E in eV ≈ 1239.84 divided by the wavelength in nanometres.
What is an electronvolt?+
One electronvolt (eV) is the energy an electron gains moving through a potential difference of one volt: exactly 1.602176634 × 10⁻¹⁹ joules. Visible photons carry roughly 1.65 to 3.26 eV.
Which colour of light has the most energy?+
Violet light has the most energy per photon among visible colours, about 3.1 eV at 400 nm, and red the least, about 1.8 eV at 700 nm. Ultraviolet, X-rays and gamma rays carry more still.
Does a brighter light have more energetic photons?+
No. Brightness (power) depends on how many photons arrive per second; each photon's energy depends only on its frequency. That is why dim UV light can cause a photoelectric effect that bright red light cannot.
How many photons does a laser pointer emit?+
Divide the power by the energy per photon. A 5 mW green laser at 532 nm emits about 1.3 × 10¹⁶ photons per second, and a 5 mW red one at 650 nm about 1.6 × 10¹⁶ because red photons carry less energy.