Quantitative Treatment of the Absorption Intensity

When light passes through a sample (or it is reflected by it) the transmitted radiation will be different from the incident one. The absorbed light will be therefore given by the difference between the intensity of the incident light (/q) and 

It is named transmittance T (3.2) of a solution the fraction of the incident radiation transmitted by the solution  

The absorbance A (3.3) of a solution, on the contrary, is defined by the following equation  

The minus sign is due to the fact that the light intensity is absorbed and therefore it decreases passing through the sample, while s indicates the proportionality constant typical of each substance, for each single wavelength in defined conditions of solvent and temperature, and it is named molar absorption coefficient. 

The integration of this differential equation from the intensity value of the incident light 7q, to the final one revealed by the detector 7, is equal to the absorbance A of the solution, and it yields the equation that is known with the name of Lambert-Beer law (3.7)  

---