A principle governing the intensity of transitions in the vibrational structure during an electronic transition in a molecule. The principle states that since nuclei are much heavier and move much more slowly than electrons (see Born-Oppenheimer approximation), an electronic transition occurs much more rapidly than the time required for the nuclei to respond to it. Therefore, in a diagram showing the electronic states of the molecule as a function of internuclear distance, the most intense electronic transition is represented by a vertical line. For this reason a transition obeying the Franck-Condon principle is called a vertical transition; when it occurs the relative positions of the nuclei remain unchanged. The Franck-Condon principle is named after James Franck (1882–1964), who stated it in 1925, and Edward Condon, who formulated it mathematically in terms of quantum mechanics in 1928.