\magnification=\magstep1

$$
{\rm mean\ free\ path\ }L \quad = 
          \quad { {1} \over {n_H} \thinspace \thinspace {\sigma} }
$$

$$
\eqalign {
{\rm optical\ depth\ } \tau \quad 
     &= \quad {\rm number\ of\ mean\ free\ paths\ through\ slab} \cr
     &= \quad { {D} \over {L} }
}
$$

$$
I \quad = \quad I_0 \thinspace e^{-\tau}
$$


\bye
