Photochemistry is a branch of science that has to do with chemical reactions and the reactions which happen by the means of the absorption of light waves which are also known as the photons of a particular energy. Such reactions may only be caused by radiation which measures from 800 nanometers to 200 nanometers. Apart from that, the photons also have to possess enough energy in it too so that it would be able to raise the atoms from its initial ground state to that of the excited state. Atoms or a group of atoms can undergo a certain chemical reaction much faster when these atoms are in an excited state and a rapid chemical reaction would not be able to take place if the atoms are in a ground state. Just as soon as an atom moves from ground state to an excited state, that particular atoms are said to be promoted from a bonding molecular orbital which also known as BMO to that of antibonding molecular orbital ABMO. It is also important for you to keep in mind that whenever a ray of light falls onto a specific metal surface, the amount of light that is being reflected mostly depends on the type of metal surface. Besides that, the definition of photochemistry provided by Wikipedia is the branch of chemistry that focuses on the chemical effects that are caused by light. In general, the term ‘photochemistry’ is used to define a certain chemical reaction that is produced from the absorption of the visible light, ultraviolet rays, or the radiations of the infrared. Consequently, this means that there are a lot of outcomes produced from the chemical reactions that have the potential to be affected by the light. The branch of photochemistry becomes more and more important with rapid development in solar technology as the reactions that happen between solar light and chemicals are going to produce electrons or to be exactly transferring the photons that are absorbed from the light to electrons which in turns produced electricity. There is a wide range of the concepts of applications of photochemistry that are very useful. Such an example of the applications of photochemistry can be found in the recent discovery and development of laser technology which can be used in biomedical treatment such as photodynamic therapy and photo diagnostics. So to summarize it, photochemistry is a chemical reaction that will only occur by the absorption of UV, visible or infrared light. To pursue the reaction, one of the reactants in the chemical reactions has to be in a state of excited or activated. To make the chemicals to be reacted, a light source is needed and usually, the reactions are quite slow due to low kinetic energy. Keep in mind that the activation energy barrier is overcome by the absorption of light energy. Other perfect example for this process is the production of vitamin D from lanolin which is the wool grease of sheep under a UVB light.