Q:  Using values of the effective cross section from
            the figure above, compute the mean free path
            for photons of wavelengths

               lambda = 912 Angstroms


                    sigma  approx     7 x 10^(-18)  cm^2

                    L      approx   1.4 x 10^(17)   cm  =   0.5 pc



               lambda = 100 Angstroms     (UV/X-ray boundary)

                    sigma  approx     2 x 10^(-20)  cm^2

                    L      approx     5 x 10^(19)   cm  =   16  pc



               lambda =  10 Angstroms     (soft X-rays)

                    sigma  approx     1 x 10^(-22)  cm^2

                    L      approx     1 x 10^(22)   cm  = 3200  pc