Consider a solution of a chemical species that absorbs light of a particular wavelength. We
could imagine two interesting situations. First, if we pass a beam of light of the appropriate
wavelength through a fairly dilute solution, we could imagine that the photons will encounter a
small number of the absorbing chemical species, so we might expect a high % transmittance and
a low absorbance. Alternatively, if we pass the same beam of light through a highly
concentrated solution, we could imagine that the photons will encounter a large number of the absorbing chemical species, and we might expect a low % transmittance and a high absorbance.
Thus absorbance is proportional to the concentration of the sample.