### Absorption

Photon with energy is absorbed by the atom in the state of n. At the same time the atom is excited from the state n to n. The number of this transition is proportional to the number of atoms in the state n, , the number of photons passing and a cross-section of this absorption. This is written as
 (2.135)

where represents the average specific intensity of radiation with corresponding wavelength. This coefficient is called Einstein's coefficient for absorption.

Kohji Tomisaka 2009-12-10