When chopped light impinges on a solid in an enclosed cell, an acoustic signal  is  produced within the cell. This effect  is  the basis of a new spectroscopic technique for the study of solid and semisolid matter. A quantitative derivation is presented for the acoustic signal in a photoacoustic cell in terms of the optical. thermal, and geometric parameters  of  the system. The theory predicts the dependence of the signal on the absorption coefficient  of  the solid, thereby giving a theoretical foundation for the technique of photoacoustic spectroscopy. In particular, the theory accounts for the experimental observation that with this technique optical absorption spectra can be obtained for materials that are optically opaque.