A photon’s energy in electronvolts is 1240 divided by its wavelength in nanometres. Green light at 550 nm is 2.25 eV.
E = hc/λ, and hc works out to 1239.84 eV·nm. Since the 2019 SI redefinition both h and c are exact by definition, so this is not a measurement that might be revised — 1240 is just a rounding, good to one part in 1,300.
It takes the VACUUM wavelength. Inside glass the wavelength shortens by the refractive index while the energy does not change at all, so feeding the in-glass figure to 1240/λ returns an energy the photon never had — carry the frequency across a boundary instead, because frequency is what stays fixed. And it is per photon: a 1 mW laser and a 1 mW transmitter deliver identical power, and nothing about that total says what one photon can do on arrival.
Estimate with the rule, then check it against the calculator that models it properly.
Open Electromagnetic Spectrum Explorer →A photon’s energy in electronvolts is 1240 divided by its wavelength in nanometres. Green light at 550 nm is 2.25 eV. E = hc/λ, and hc works out to 1239.84 eV·nm.