UV spectroscopy involves the absorption of ultraviolet or visible light by molecules, causing electronic transitions from ground state to excited states. The absorption is measured using instruments like spectrophotometers and plotted against concentration to verify Beer's law. Deviations from Beer's law can occur due to chemical effects like association/dissociation. Instruments consist of sources, filters/monochromators, sample cells, and detectors like photovoltaic cells, phototubes, and photomultiplier tubes which generate a current proportional to light intensity and wavelength.