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The relationships among various parameters describing the strength of opticaltransitions in atoms and molecules are reviewed. The application of theseparameters to the description of the interaction between nearly monochromatic,directional light beams and atoms and molecules is given careful attention.Common pitfalls in relating these parameters are pointed out. This is a revised(February, 2002) version of a paper that originally appeared in Am. J. Phys.50, 982-986 (1982).

Einstein coefficients, cross sections, f values, dipole moments, and all that