26 OPTICS FOR ENGINEERS
remains useful for discussing absorption and emission of light. The reader may nd a book by
Hoffmann [73] of interest, both for its very readable account of the development of quantum
mechanics and for the titles of chapters describing Bohr’s theory, “The Atom of Niels Bohr,”
and its subsequent replacement by more modern theories, “The Atom of Bohr Kneels.”
Although we are almost always interested in the interaction of light with more complicated
materials than hydrogen atoms, all materials can be described by an energy ladder. Figure 1.8A
shows the process of absorption, followed by decay to an intermediate state and emission of
a photon. The incident photon must have an energy matching the energy difference between
the