Photosynthesis is chemically a redox reaction in which water is oxidised and carbon dioxide is reduced. The oxidation of water is called as photolysis and is responsible for the splitting of water molecule to release oxygen molecule.
The experimental proof was provided by, Ruben, Hassid and Kamen, who used water containing heavy isotope of oxygen i.e., O−18, whereas carbon dioxide containing normal oxygen, in the process of photosynthesis. The oxygen released in the process of photosynthesis was heavier oxygen i.e., O−18.