Beer-Lambert Law is used (which can also be called “Beer’s Law”) in order to calculate concentration from absorbance. The amount of light absorbed is proportional to the length of the light path (l). Practically, this is the container, usually a cuvette, in which the material in question is held. The relationship between absorbance and concentration (c) is proportional.
The equation for Beer’s law is: A = εmCl
m = (εm) = slope or the molar extinction coefficient in beers law which has units of M−1cm−1
(A=absorbance, εm = molar extinction coefficient, C = concentration, l=path length of 1 cm)