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Photons interact with matter because matter contains electric charges. The electric field of light exerts forces on the electric charges and dipoles in atoms, molecules, and solids, causing them to vibrate or accelerate. Conversely, | Fundamentals of Photonics Bahaa E. A. Saleh Malvin Carl Teich Copyright 1991 John Wiley Sons Inc. ISBNs 0-471-83965-5 Hardback 0-471-2-1374-8 Electronic CHAPTER 12 PHOTONS AND ATOMS ATOMS MOLECULES AND SOLIDS A. Energy Levels B. Occupation of Energy Levels in Thermal Equilibrium INTERACTIONS OF PHOTONS WITH ATOMS A. Interaction of Single-Mode Light with an Atom B. Spontaneous Emission C. Stimulated Emission and Absorption D. Line Broadening E. Laser Cooling and Trapping of Atoms THERMAL LIGHT A. Thermal Equilibrium Between Photons and Atoms B. Blackbody Radiation Spectrum LUMINESCENCE LIGHT Niels Bohr 1885-1962 Albert Einstein 1879-1955 Bohr and Einstein laid the theoretical foundations for describing the interaction of light with matter. 423 Photons interact with matter because matter contains electric charges. The electric field of light exerts forces on the electric charges and dipoles in atoms molecules and solids causing them to vibrate or accelerate. Conversely vibrating electric charges emit light. Atoms molecules and solids have specific allowed energy levels determined by the rules of quantum mechanics. Light interacts with an atom through changes in the potential energy arising from forces on the electric charges induced by the time-varying electric field of the light. A photon may interact with an atom if its energy matches the difference between two energy levels. The photon may impart its energy to the atom raising it to a higher energy level. The photon is then said to be absorbed or annihilated . An alternative process can also occur. The atom can undergo a transition to a lower energy level resulting in the emission or creation of a photon of energy equal to the difference between the energy levels. Matter constantly undergoes upward and downward transitions among its allowed energy levels. Some of these transitions are caused by thermal excitations and lead to photon emission and absorption. The result is the generation of .