Absorption of Light and Deviation from the Beer-Lambert Law

ABSORPTION OF LIGHT AND DEVIATION FROM THE BEER-LAMBERT LAW 

A fundamoital aspect of fluores nce spectroscopy is the measurement of light absorption. While the theory of light absoiptim is wdl known a number of factors can result in misleadhig measumuMts of U t d s(xption. We will first derive the BecT -Lambert law which aUaws one to describe the reasons for deviations from this law. 

Rearrangement and integration, subject to the boundaiy condition/=Jb St jc=(X yidd 

Since n = NdVf, where N is Avogadro s Immber, we obtain 

It is interesting to calculate the absorption cross section for typical aromatic compounds. The extinction coefficients of anthracene are 160,0(X and6300Ar cm at2S3 and 375 nm, respectively. These values correspond to cross sections of 6.1 and 0.24 respectively. Assuming the molecular cross section of anthracene to be 12 A we see that anthracene absorbs about 50% of the photons it encounters at 253 nm and 2% of the photons at 375 nm. 

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