When monochromatic radiation (it can be UV rays) is passed through a medium, the intensity of the transmitted radiation decreases with the increase in the thickness of the absorbing medium, and it varies directly with the incident radiation.
Mathematically, we can express this statement as:
(Here, the negative sign indicates the decrease in the intensity of the transmitted radiations)
Equation says that the rate of decrease in the intensity to the thickness is directly proportional to the incident radiation.equation (1) can be rewritten as:
Here, It = Intensity of transmitted radiation k’ = Proportionality constant.
Equation 1 can be written in this form also:
This expression says that the absorbance of light in a homogenous material/medium is directly proportional to the thickness of the material/medium.
So, A = εb