According to the **Beer-Lambert Law**,
*A* = *e* × *b* × *c*
Where *A* is the absorbance of a sample, *e* is the extinction coefficient
of the substance in the sample, *b* is the path length of the sample (in
cm) and *c* is the concentration of the compound in solution in the
sample.
So, in order to find the **dye surface concentration** (not just the
concentration of the dye in the solution), the authors divide the
absorbance of the sample (2.45, unitless) by the extinction coefficient
of the dye. This yields a number of the form
*bc* = *length* × *concentration* = (*cm*) × (*mol*/*cm*^{3}) = *mol*/*cm*^{2} = *surface* *concentration*
Pretty tricky.