We saw that the process of photon absorption by the atom is a process of raising the atom (electron) from a lower energy level into a higher energy level (excited state), by an amount of energy which is equivalent to the energy of the absorbed photon.
Our discussion involved a microscopic system in which one photon interacts with one atom.
In a macroscopic system, when electromagnetic radiation passes through matter, part of it is transmitted, and part is absorbed by the atoms.
The intensity (I) of the transmitted radiation through a thickness (x) of homogeneous material, is described by the experimental equation of exponential absorption (Lambert Law):
The transmission (T) of this material is described by the relation between the transmitted intensity (I) to the incident intensity (I0):
From the last two equations we get the Transmission:
It is common to use units of centimeter (10-2 [m]), to measure the width of the material (x), so the units of the absorption coefficient (a) are:
Every material is transparent differently to different wavelengths,
so the absorption coefficient (a)
a function of the wavelength: a(l).
This fact is very important (as we shall see) to understand the interaction
of electromagnetic radiation with matter, in the variety of applications
of the laser.