Topic overview

Start with the big picture

This topic introduces the pathways by which light energy is absorbed and produces biological effects. Chromophores absorb at characteristic wavelengths, and their action spectrum relates wavelength to biological response. The Jablonski diagram organizes electronic states and transitions, including fluorescence and phosphorescence. In photosensitization, Type I and Type II pathways generate reactive oxygen species; singlet oxygen is important in photodynamic therapy. The lesson also addresses photobleaching, reaction quantum yield, and use of the Beer–Lambert law to estimate chromophore concentration, with tissue scattering as a complicating factor. Biological consequences and defenses include DNA photoproduct formation, photoprotection and repair. The chapter further surveys UV effects, photomorphogenesis, optogenetics, blue-light hazard, therapeutic photobiology, and experimental design.