Photochemistry, a sub-discipline of chemistry, is the study of the interactions between atoms, small molecules, and light (or electromagnetic radiation). Like most scientific disciplines, photochemistry utilises the SI or metric measurement system. Important units and constants that show up regularly include the meter (and variants such as centimetre, millimetre, micrometer, manometer, etc.), seconds, hertz, joules, moles, the gas constant R, and the Boltzmann constant. These units and constants are also integral to the field of physical chemistry.There are important processes based in the...