Light absorption Beers law

Equation (24.7) states that the relative decrease in intensity of the beam is proportional to the number of absorbing molecules in the slab of material. If there are several kinds of 

The constants, /c2. are characteristic of the substances in question. For any substance the value of k depends on the wavelength. If a substance is transparent at a particular wavelength, all the light goes through and /c = 0. If at a particular wavelength all the substances are transparent except one, then Eq. (24.8) reduces to Eq. (24.7). Integration of Eq. (24.7) yields 

It is customary in spectrophotometry to use common logarithms rather than natural logarithms thus in Eq. (24.9) we replace the natural base, e, by io 3429 - obtain I = Io We define e = 0.4343fc then 

The constant, 6, is the molar absorption coefficient of the substance e is also called the extinction coefficient.The transmittance, T, is defined by 

If the absorbance increases by unity, the transmittance drops by a factor of ten. Equation (24.10) is an expression of the Beer-Lambert law, often called simply Beer s law. Beer s law is the basic equation for the various colorimetric and spectrophotometric methods of analysis. If Beer s law holds, then the absorbance is given by 

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