Time-dependent quantum mechanics shows how mixing orbitals of different energy causes electrons to vibrate. Mixing 1s with 2p causes a vibration that can absorb or generate light, while mixing 1s with 2s causes "breathing" that does not interact with light. Many natural organic chromophores involve mixing an unshared electron pair with a vacant pi orbital, whose conjugation determines color. Infrared spectra reveal atomic vibration frequencies, which are related by Hooke’s law to bond strengths and "reduced" masses. Infrared spectra are complicated by the coupling of local oscillators of similar frequency to give "normal" modes. Alkane chains possess characteristic stretching and bending modes, with descriptive names, that may, or may not, absorb infrared light.