FTP, name this law named for either one or a pair of German scientists that states that the absorbance of a medium is equal to the product of its absorptivity, path length, and absorber concentration.
The naive form of this statement may be amended by the inclusion of a buildup factor in the case that the medium length or beam diameter is non-negligible; it must be suitably modified with a transfer coefficient if interference can change the incident wavelength.  This law's major factor is essentially equivalent to the medium's macroscopic cross-section, which is turn directly proportional to the interaction probability per unit path length.  If the incident flux is of monochromatic light is in the appropriate concentration range, this spectroscopic law is generally highly accurate.  FTP, name this law named for either one or a pair of German scientists that states that the absorbance of a medium is equal to the product of its absorptivity, path length, and absorber concentration.
The naive form of this statement may be amended by the inclusion of a buildup factor in the case that the medium length or beam diameter is non-negligible; it must be suitably modified with a transfer coefficient if interference can change the incident wavelength.  
Lambert-Beer law (or Beer-Lambert law or Beer's law)