A = ε × l × c
The Beer-Lambert law states that absorbance is directly proportional to the concentration of the absorbing species and the path length of light through the sample: A = εlc. ε (molar absorptivity or extinction coefficient) is a constant specific to each substance at each wavelength, measured in L/(mol·cm). Standard cuvettes have l = 1 cm. Transmittance T = 10^(-A), so A = 1 means 10% transmittance, A = 2 means 1%. The law is linear for A < 1.5; above this, deviations occur due to molecular interactions and stray light. UV-Vis spectrophotometry using this law is the most common analytical technique in chemistry labs, used for determining concentrations of colored solutions, DNA/protein quantification, and water quality analysis.