R. Hill is credited with discovering that oxygen is evolved from water during photosynthesis. This discovery was made through a series of experiments conducted by Hill and his colleagues in the 1930s.

Background:
Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, stored in the form of glucose. It is a complex process that occurs in the chloroplasts of these organisms.

Experiment:
Hill's experiments aimed to elucidate the mechanism of photosynthesis, specifically the source of oxygen produced during the process. To do this, he used isolated chloroplasts and an artificial electron acceptor called ferricyanide.

Procedure:
1. Hill and his colleagues exposed the isolated chloroplasts to light, causing them to undergo photosynthesis.
2. They then added the ferricyanide to the system.
3. The ferricyanide acted as an electron acceptor, accepting electrons from the photosynthetic electron transport chain.
4. In the presence of light, the electrons were transferred from water to the electron transport chain, reducing the ferricyanide to ferrocyanide.
5. The reduction of ferricyanide was accompanied by the evolution of oxygen gas.

Results:
Hill observed that the evolution of oxygen occurred only when the chloroplasts were exposed to light and ferricyanide. This led him to conclude that oxygen is produced from the splitting of water during photosynthesis. The electrons released from water during this process are used to reduce the ferricyanide and generate oxygen gas as a byproduct.

Significance:
Hill's discovery of oxygen evolution from water in photosynthesis was a significant contribution to our understanding of the process. It provided experimental evidence for the light-dependent reaction of photosynthesis, which involves the splitting of water and the release of oxygen. This discovery laid the foundation for further research on the mechanism of photosynthesis and the role of water in the process.