Photon Energy Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
A photon's energy is calculated with E = h × c ÷ λ, where h is Planck's constant and c the speed of light. A green light photon (500 nm) has an energy of about 2.48 electronvolts (eV).
Explanation
In quantum physics, light behaves both as a wave and as a stream of particles called photons, each carrying a precise amount of energy directly linked to its wavelength (or, equivalently, its frequency — see our wavelength and frequency calculator for this relationship). The shorter the wavelength, the higher the photon's energy: this is why violet light (shorter wavelength, around 400 nm) carries more energy per photon than red light (longer wavelength, around 700 nm), and why ultraviolet rays, even shorter than visible violet, carry enough energy per photon to damage DNA and cause sunburn, unlike visible light or infrared. A photon's energy is expressed in joules in the International System, but this unit gives extremely small values at the scale of a single photon (on the order of 10⁻¹⁹ joules for visible light); the electronvolt (eV), an energy unit suited to the atomic scale, is therefore more commonly used in physics and chemistry to express this kind of energy more legibly. Planck's constant and the speed of light are both exact values by definition since the 2019 redefinition of the International System of Units, which makes this calculation entirely deterministic, with no measurement uncertainty on the constants themselves. A sufficiently intense and hot source emits an entire spectrum of photons of different energies, described by our Stefan-Boltzmann law calculator for the total radiated power.
Example: a green light photon, 500 nm
Inputs
Wavelength: 500 nm.
Calculation
E = (6.62607015×10⁻³⁴ × 299,792,458) ÷ (500×10⁻⁹) ≈ 3.973×10⁻¹⁹ J. Converted to electronvolts: 3.973×10⁻¹⁹ ÷ 1.602176634×10⁻¹⁹ ≈ 2.4797 eV.
Result
A green light photon at 500 nm carries an energy of about 2.48 eV.
Frequently asked questions
Why is ultraviolet light more dangerous than visible light?
Because its wavelength, shorter than visible light, gives it a higher energy per photon, in some cases enough to damage chemical bonds in biological molecules, notably the DNA of skin cells. This mechanism is what explains the role of ultraviolet light in sunburn and the increased skin cancer risk linked to excessive, unprotected exposure.
Why use the electronvolt rather than the joule for a photon's energy?
Because the joule, although the International System's unit of energy, gives extremely small and hard-to-read numeric values at the scale of a single photon (on the order of 10⁻¹⁹ for visible light). The electronvolt, defined as the energy gained by an electron accelerated through a potential difference of one volt, gives values on the order of unity for visible photons, much more practical to handle and compare.
Does this formula also apply to radio waves and X-rays?
Yes, the relationship E = hc/λ applies across the entire electromagnetic spectrum, from radio waves (long wavelengths, very low energy per photon) to gamma rays (extremely short wavelengths, very high energy per photon), passing through visible light. Only the scale of the values changes considerably from one end of the spectrum to the other.