The band gap energy of silicon is 1.26 eV.  Photons with
   less energy cannot knock an electron free from an atom.


   Q:  What is the wavelength of a photon corresponding
          to this critical energy?



       The energy of a photon is

                                 h * c
               E  =  h * nu  =  ---------
                                 lambda


       where

                 E   =  energy

                                                           -34
                 h   =  Planck's constant  =     6.626 x 10    J*s

                                                     8
                 c   =  speed of light     =   3 x 10  m/s

             lambda  =  wavelength of light


       There is a nice combination of (h*c) which is convenient:

                   (h * c)  =   1240 eV * nm


       That means

                             1240 eV * nm
                     E    =  ------------
                                lambda


       or, in this case,


                             (h * c)      1240 eV*nm 
                  lambda  =  -------  =  ----------   =   984 nm
                                E          1.26 nm