Photochemistry

In the early 1800s Christian von Grotthus (1785-1822) and John Draper (1811-1882) formulated the first law of photochemistry, which states that only light that is absorbed by a molecule can produce a photochemical change in that molecule. This law relates photochemical activity to the fact that each chemical substance absorbs only certain wavelengths of light, the set of which is unique to that substance. Therefore, the presence of light alone is not sufficient to induce a photochemical reaction; the light must also be of the correct wavelength to be absorbed by the reactant species.

In the early 1900s the development of the quantum theory of light—the idea that light is absorbed in discrete packets of energy called photons—led to the extension of the laws of photochemistry. The second law of photo-chemistry, developed by Johannes Stark (1874-1957) and Albert Einstein (1879-1955), states that only one quantum, or one photon, of light is absorbed by each molecule undergoing a photochemical reaction. In other words, there is a one-to-one correspondence between the number of absorbed photons and the number of excited species. The ability to accurately determine the number of photons leading to a reaction enables the efficiency, or quantum yield, of the reaction to be calculated.