Bouguer-Lambert-Beer law

So:

 The absorbance of a solution is proportional to concentration of light-absorbing substance and a thickness of a layer

Or

 The relationship between a sample’s absorbance and the concentration of the absorbing species

where: A – optical density (absorbance), ε – the molar absorptivity, C – concentration (molarity)

Additivity of optical densities

Beer’s law can be extended to samples containing several absorbing components provided that there are no interactions between the components. Individual absorbances, A_i, are additive. For a two-component mixture of X and Y, the total absorbance, A _tot, is

So

A = l(e₁С₁ + e₂С₂ + …e_kС_k)