Rami Arieli: " The
Laser Adventure" Chapter 2.3 page 1

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 (I_{0}):

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)
is
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.