According to Lambert-Beer law, when a beam of monochromatic radiation passes through any solution, the intensity of the beam reduces to some amount. Let if I_{0} is the intensity of incident beam and I_{t} is the intensity of transmitted beam, then the intensity of energy absorbed I_{a} is given by

I_{a} = I_{0} – I_{t}

Also the amount of light intensity absorbed by the sample is directly proportional to the concentration and the thickness of the absorption material. i.e.

dI/I = – acdx …..(2)

Where dI is change in intensity after travelling through the sample, dx is the thickness of the absorption sample with concentration c and a is the proportionality constant. Where minus sign indicated that there is decrease in intensity. Integration Eq. 2 between I = I_{0} to I = I at x = b gives

In(I/I_{0}) = 2.303 log(I/I_{0}) = – abc …….(3)

The amount of intensity, when passing through any sample decreases exponentially with increase in thickness of the sample and the concentration of the medium. This phenomenon of decrease in intensity with medium and concentration is called Lambert-Beer law.

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