Q:  What is the mean free path of light inside this cloud?

                         1                        1
         mfp L  =  -------------   =  ----------------------------------
                    n * (pi r^2)       10^(-3) m^(-3) * pi * (2 x 10^(-7) m)^2


                =   7.9 x 10^(15)  m


  Q:  What is the optical depth of the cloud?

          Since the cloud is D = 1 pc thick, the optical depth is
          the ratio of thickness to mean free path:

                           D          3.08 x 10^(16) m
               tau  =   --------  =  -----------------  =  3.88
                           L          7.9 x 10^(15) m


  Q:  How many magnitudes of extinction will a star suffer
            as its light must pass through this cloud?


            The intensity of starlight will decrease to a value


                 I         -tau         -3.88
                ----   =  e        =   e        =   0.0206
                 I0


            so the difference in magnitude will be

 
                 mag - mag0  =  -2.5 log10 (0.0206)  =  4.2 mag


            So a star will appear fainter by 4.2 magnitudes if 
            its light must pass through the cloud to reach us.