A = Ecl, where
A = absorbance (optical density, OD); equal to –log (I/Io); A is unitless
Io = intensity of light irradiating the sample
I = intensity of light transmitted through the sample
E = absorption coefficient or absorptivity; a constant that reflects the efficiency or the extent of absorption at selected wavelengths. E is dependent on the wavelengths of light and on the chemical nature of the substance involved). Formerly, e was referred to as the extinction coefficient. For biological molecules, E is often expressed as the molar absorption coefficient, e, which is the absorbance of a 1.0 M solution of pure absorbing material in a 1 cm cuvette with given wavelength and given solvent. The units of E are M-1cm-1.

l = length of the path of light through the sample or the thickness of the cell (cuvette)

c = concentration of the substance

Calculate the molar concentration of a solution of the nucleotide base uracil. A of
uracil at 260 nm is 0.65; A of pure solvent in a matched 1 cm cuvette is 0.07. e is 8.2
x103 M-1cm-1. (A = (absorbance of solvent + sample) – (absorbance of solvent))