Robin Hill experiment: He took the isolated chloroplasts of Stellaria and placed them in water having hydrogen acceptors (like chromates, ferricyanides, cytochromes, benzoquinones etc.) He illuminated water and the oxygen was liberated in the absence of carbondioxide.
The hydrogen acceptors are called Hill oxidants and the process of liberation of oxygen in the absence of carbon dioxide is called Hill oxidation. In plants hydrogen acceptor is NADP+
Van Niel’s observation : Van Niel observed that photosynthesis is a light-dependent reaction and the O2 comes from the water and not carbon dioxide.. In green plants H2O is the hydrogen donor and O2 is the product. whereas in purple and green bacteria the hydrogen donor is H2S and the product is sulphur.