Light absorbance absorption, transmission

If less light remains following transmission through a sample then, from Equation (9.7), I < /c, and A > 1. It should be clear that the absorbance is zero when the same number of photons enter the sample as leave it. A highly coloured sample is characterized by a high absorption, so A is large. 

If light-absorbing substances are mixed with the standard white or if the substances are present as absorbing spots on the surface of such a standard, the intensity of remission will be weakened this weakening occurs in those wave length regions where absorption bands would occur in transmission measurements on solutions of the substances. The remission can be calculated quantitatively with the help of the Kubelka-Munk function. 

An interferometric method was first used by Porter and Topp [1, 92] to perfonn a time-resolved absorption experiment with a -switched ruby laser in the 1960s. The nonlinear crystal in the autocorrelation apparatus shown in figure B2.T2 is replaced by an absorbing sample, and then tlie transmission of the variably delayed pulse of light is measured as a fiinction of the delay This approach is known today as a pump-probe experiment the first pulse to arrive at the sample transfers (pumps) molecules to an excited energy level and the delayed pulse probes the population (and, possibly, the coherence) so prepared as a fiinction of time. 

Beer s Law. We have so far considered the light absorption and the light transmission for monochromatic light as a function of the thickness of the absorbing layer only. In quantitative analysis, however, we are mainly concerned with solutions. Beer studied the effect of concentration of the coloured constituent in solution upon the light transmission or absorption. He found the same relation between transmission and concentration as Lambert had discovered between transmission and thickness of the layer [equation (3)], i.e. the intensity of a beam of monochromatic light decreases exponentially as the concentration of the absorbing substance increases arithmetically. This may be written in the form ... 

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