Michael McBride continues Yale's Freshman Organic Chemistry II with a lecture on how time-dependent quantum mechanics explains spectroscopy. He shows how mixing orbitals of different energy, such as 1s with 2p, produces electron vibration that can absorb or emit light, while 1s to 2s mixing gives a non-interacting breathing motion. The lecture connects organic chromophores to conjugation between unshared electron pairs and vacant pi orbitals, then turns to infrared spectroscopy, using Hooke's law to relate vibration frequency to bond strength and reduced mass. McBride explains why real IR spectra are complicated by coupling between local oscillators of similar frequency, producing normal modes, and works through the characteristic stretching and bending motions of alkane chains. Recorded in spring 2011 as part of Open Yale Courses, the lecture runs about fifty minutes and is organized into four chapters covering electronic spectroscopy, chromophores, Hooke's law, and coupled oscillators.