What is the deBroglie wavelength of a neutron whose kinetic energy is 0.04 eV? Help: We give masses in terms of their mc2 energy equivalent, because that simplifies the numerical calculation when energies are given in eV. By putting factors of c in the appropriate places, you can avoid having to transform to and from SI units. This is the reason why the constant, hc = 1240 eV.nm, is useful. However, if you find this confusing, just put everything into SI units.