Q:  What is the energy of a photon in the middle of the optical range,
            with a wavelength of 5000 Angstroms?


                   hc          (6.636 x 10^(-34) J*s) (3 x 10^(8) m/s)
          E  =  --------   =  ----------------------------------------
                 lambda                 5000 x 10^(-10) m


             =  3.97 x 10^(-19) Joules

             =  3.97 x 10^(-12) ergs

                                                1 eV
             =  3.97 x 10^(-19) Joules *  ------------------------
                                           1.602 x 10^(-19) Joules

             =  2.48 eV


       For optical photons, eV are a convenient unit.