what is meant by photochemical reaction
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Answer:
Photochemical reaction, a chemical reaction initiated by the absorption of energy in the form of light. The consequence of molecules’ absorbing light is the creation of transient excited states whose chemical and physical properties differ greatly from the original molecules. These new chemical species can fall apart, change to new structures, combine with each other or other molecules, or transfer electrons, hydrogen atoms, protons, or their electronic excitation energy to other molecules. Excited states are stronger acids and stronger reductants than the original ground states.


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Related Topics:
photosynthesis Photochemical equivalence law Photosensitization Photolysis Quantum yield
It is this last property that is crucial in the most important of all photochemical processes, photosynthesis, upon which almost all life on Earth depends. Through photosynthesis, plants convert the energy of sunlight into stored chemical energy by forming carbohydrates from atmospheric carbon dioxide and water and releasing molecular oxygen as a byproduct. Both carbohydrates and oxygen are needed to sustain animal life. Many other processes in nature are photochemical. The ability to see the world starts with a photochemical reaction in the eye, in which retinal, a molecule in the photoreceptor cell rhodopsin, isomerizes (or changes shape) about a double bond after absorbing light. Vitamin D, essential for normal bone and teeth development and kidney function, is formed in the skin of animals after exposure of the chemical 7-dehydrocholesterol to sunlight. Ozone protects Earth’s surface from intense, deep ultraviolet (UV) irradiation, which is damaging to DNA and is formed in the stratosphere by a photochemical dissociation (separation) of molecular oxygen (O2) into individual oxygen atoms, followed by subsequent reaction of those oxygen atoms with molecular oxygen to produce ozone (O3). UV radiation that does get through the ozone layer photochemically damages DNA, which in turn introduces mutations on its replication that can lead to skin cancer.

ozone depletion
Antarctic ozone hole, September 17, 2001.
NASA/Goddard Space Flight Center

BRITANNICA QUIZ
Ins and Outs of Chemistry
You may know that elements make up the air we breathe and the water we drink, but do you know more about them? What element is almost as light as hydrogen? What do you call a blend of two chemical elements? Find out the answers in this quiz.
Photochemical reactions and the properties of excited states are also critical in many commercial processes and devices. Photography and xerography are both based upon photochemical processes, while the manufacture of semiconductor chips or the preparation of masks for printing newspapers relies on UV light to destroy molecules in selected regions of polymer masks.

The sequence of operations in making one type of integrated circuit, or microchip, called an n-channel (containing free electrons) metal-oxide semiconductor transistor. First, a clean p-type (containing positively charged “holes”) silicon wafer is oxidized to produce a thin layer of silicon dioxide and is coated with a radiation-sensitive film called a resist (a). The wafer is masked by lithography to expose it selectively to ultraviolet light, which causes the resist to become soluble (b). Light-exposed areas are dissolved, exposing parts of the silicon dioxide layer, which are removed by an etching process (c). The remaining resist material is removed in a liquid bath. The areas of silicon exposed by the etching process are changed from p-type (pink) to n-type (yellow) by exposure to either arsenic or phosphorus vapour at high temperatures (d). Areas covered by silicon dioxide remain p-type. The silicon dioxide is removed (e), and the wafer is oxidized again (f). An opening is etched down to the p-type silicon, using a reverse mask with the lithography-etching process (g). Another oxidation cycle forms a thin layer of silicon dioxide on the p-type region of the wafer (h). Windows are etched in the n-type silicon areas in preparation for metal deposits (i).
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History
The use of photochemistry by humans began in the late Bronze Age by 1500 BCE when Canaanite peoples settled the eastern coastline of the Mediterranean. They prepared a purple fast dye (now called 6,6’-dibromoindigotin) from a local mollusk, using a photochemical reaction, and its use was later mentioned in Iron Age documents that described earlier times, such as the epics of Homer and the Pentateuch. In fact, the word Canaan may mean “reddish purple.” This dye, known as Tyrian purple, was later