Book excerpt: Dynamical Properties of Solids, Volume 4: Disordered Solids, Optical Properties focuses on the lattice dynamical properties of noncrystalline and disordered solids and optical properties of crystalline solids. The selection first elaborates on the vibrational properties of amorphous solids and computer experiments and disordered solids. Topics include thermal and electrical transport, density of states, numerical methods, localization, low frequency modes, and theoretical background. The text then takes a look at the morphic effects in lattice dynamics, including normal coordinate formalism, electric-field-induced infrared absorption and Raman scattering, stress-induced changes in the phonon frequencies, and the effect of time reversal on the symmetry of the long-wavelength optical. The manuscript examines the absorption of infrared radiation by multiphonon processes in solids, as well as theoretical studies of infrared absorption in the multiphonon region and experimental studies of infrared absorption at frequencies above the characteristic lattice vibration frequencies. The selection is a dependable source of data for researchers interested in the optical properties of crystalline solids and lattice dynamical properties of noncrystalline and disordered solids.