What is Absorbance and the Beer–Lambert law?

For light of intensity $I_0$ entering a solution and $I$ leaving it, the absorbance is $A = \log(I_0/I)$. For a dilute solution of one absorbing species, $A = \varepsilon\,\ell\,c$ (the Beer–Lambert law), where $\ell$ is the path length, $c$ the concentration and $\varepsilon$ the molar absorption coefficient, a property of the species at that wavelength.