Photon Energy Calculator

Photon energy: E = h·f = h·c/λ

What fixes a photon's energy is its frequency, or equivalently its wavelength: E = h·f = h·c/λ. Here h = 6.626·10⁻³⁴ J·s is Planck's constant and c = 3·10⁸ m/s. You get the answer in joules. To read it in electronvolts (eV), divide by 1.602·10⁻¹⁹. Take a green photon in the visible range, λ = 550 nm: it carries E ≈ 3.6·10⁻¹⁹ J ≈ 2.25 eV. Push toward the ultraviolet and the energy climbs, which is exactly why UV light ionizes and damages DNA. Go the other way, into the infrared, and each photon carries less. Einstein's 1905 paper on the photoelectric effect (Nobel 1921) made the point clear: only photons above the material's work function can knock electrons loose. Light comes in discrete chunks, not a continuous stream.

Applications

Solar panels, where the silicon band gap of about 1.1 eV sets the cutoff wavelength. Spectroscopy, with the Balmer lines from hydrogen. LEDs, whose emitted color tracks the semiconductor's band gap. Then there's quantum electrodynamics (QED), CCD and CMOS image sensors, and lasers, which rely on stimulated emission at a fixed E.

FAQ

Why are UV photons more dangerous than visible light? A shorter λ means a higher E. UV photons pack enough energy, above 3 eV, to break molecular bonds and ionize atoms, which is how they damage DNA.

How do I convert joules to eV? Just divide by 1.602·10⁻¹⁹. For instance, 3.6·10⁻¹⁹ J ÷ 1.602·10⁻¹⁹ ≈ 2.25 eV.

Does a more intense light beam have more energetic photons? No. A brighter beam just means more photons arriving each second. Frequency alone decides how much energy any single photon carries. That was Einstein's key insight in the photoelectric effect.