Absorbance is related to the number of photons a sample absorbs. When an atom or molecule absorbs a single photon, an electron makes a transition to a higher orbital energy. As the number of atoms or molecules increases more photons can be absorbed. The number of photons a sample absorbs is directly related to the number of atoms or molecules that are present in the sample.
Absorbance (A) is defined as the ratio of the number of photons before and after the light beam passes through a sample: A=log(P0/P)
Where P0 = number of photons before and P = the number of photons after.
This absorbance is directly proportional to the concentration, c, of the light-absorbing atoms or molecules in a sample. This relationship (known as the Beer-Lambert or Beer’s Law) is: A=εlc
Where A is the absorbance (a dimensionless value – typically given the unit of ‘absorbance units’ or AU), c is the concentration (typically in moles per liter), l is the pathlength or distance the light travels through the sample, and ε (epsilon) is the molar absorptivity (M-1cm-1).
