The Beer-Lambert Law (Equation1) is the principle behind absorbance spectroscopy. For a single wavelength, A is absorbance (unit less, usually seen as arb. units or arbitrary units), is the molar absorptivity of the compound or molecule in solution (M-1cm-1), b is the path length of the cuvette or sample holder (usually 1 cm), and c is the concentration of the solution (M).