Why is absorbance directly proportional to concentration?

Using the Beer-Lambert Law, we can mathematically show that absorbance is directly proportional to the concentration of the solution of the sample. The Beer-Lambert Law is expressed by the following equation:

Where:

• A = the molar absorptivity
• ε = the molar extinction coefficient
• l= fixed path length
• c= the concentration

In practice, ε and l are constants, and A and c are variables. Therefore, if the concentration (c) increases or decreases at a certain rate then the absorbance (A) increases or decreases at the same rate (i.e. they are proportional to one another)