The absorbance per centimetre of path length when the concentration of absorbing material is 1 M; = A/(bc) where a, A, b, and c are the molar absorptivity, absorbance, path length in cm, and concentration in mol/L, respectively. Molar absorptivities are sometimes written as absorption cross sections by expressing the concentration in units of mol/cm3, so that the molar absorptivity takes on units of cm2/mol.