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 (1902–74), who formulated it mathematically in terms of quantum mechanics in 1928.