Q:  What is the number density of photons per cubic meter
             inside the box?

                                1.78 x 10^(26) photons
          number density  =  ------------------------------
                                  3 x 10^(8) cubic meters


                          =   5.93 x 10^(17)   photons per cubic meter


  Q:  What is the energy density of the photons inside the box?


           Each photon has energy 3.61 x 10^(-19) Joules, 
           so the amount of energy inside a single cubic meter is

                                              photons
        energy density   =   5.93 x 10^(17) ------------ * 3.61 x 10^(-19) J
                                             cubic meter


                             =   0.2  Joules per cubic meter


            Close enough to the actual value of 0.85 J/m^3.