Probability of transitions. The Beer-Lambert Law. Oscillator strength

Probability of transitions. The Beer-Lambert Law. Oscillator strength 

Experimentally, the efficiency of light absorption at a wavelength X by an absorbing medium is characterized by the absorbance A(X) or the transmittance T(X), defined as 

In many cases, the absorbance of a sample follows the Beer-Lambert Law 

2) The term intensity is commonly used but is imprecise. According to IUPAC recommendations (see Pure 6[ Appl. Chem. 68, 2223-2286 (1996)), this term should be replaced by the spectral radiant power Ph i.e. the radiant power at wavelength X per unit wavelength 

Failure to obey the linear dependence of the absorbance on concentration, according to the Beer-Lambert Law, may be due to aggregate formation at high concentrations or to the presence of other absorbing species. 

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