Complex sentence: They were using Einstein's proposed relationship between the energy of a photon (a particle of light) and the corresponding wavelength of light, which says that photons of light with a 254 nanometer (nm) wavelength have 4.9 eV of energy.
Split into simpler sentences: They were using Einstein's proposed relationship between the energy of a photon (a particle of light) and the corresponding wavelength of light. This relationship says that photons of light with a 254 nanometer (nm) wavelength have 4.9 eV of energy.