Consider a beam of parallel monochromatic light with intensity I0 striking the sample at the surface as shown in the figure above, such that it is perpendicular to the surface. After passing through the path length b of the sample, which contains N molecules/cm3, the intensity of the light reduces to IT.
Now, consider a cross-section of the block having an area S and an infinitesimal thickness dz placed at a distance z from the surface.