eV <-> nm
(Depending on what’s your business,) quite often you want to calculate a photon’s energy E (in electron volt, eV) from it’s wavelength λ (in nm) or vice versa. If you throw all together (charge of electron, speed of light, Planck’s constant, decades) you just get
1240 = E [eV]*λ[nm]
I’m sure everyone to whom this might be helpful has memorized this shortcut already. I’m just writing it down here so that I won’t forget the 1240 myself anymore. 1240, 1240, 1240….