Molecules all have particular resonance frequencies. The tendency for molecules to absorb and emit energy as a function of frequency can be measured and this is spectroscopy. Electronic spectra provide information about the electronic properties of molecules. The interest of the chemist in an excited molecule often begins exactly where the physicist ends. Beyond spectroscopy lies the vast, complex, and vitally important field of photochemistry.
Visual reception depends upon photochemical reactions of the visual pigments.
Apart from such biochemical applications, photochemistry is a subject pursued by many organic and inorganic chemists, who have discovered that intricate synthesis and rearrangements of molecules can be produced by light, which would be difficult or impossible by ordinary thermal reactions.