How does light intensity affect the rate of photosynthesis?
Photosynthesis requires water, carbon dioxide and light. Even if there is a very good supply of water and carbon dioxide there will be a maximum rate reached when the enzyme that catalyses the conversion from inorganic to organic carbon (ribulose-1,5-bisphosphatephosphate or 'rubisco') is saturated. At very high light intensities the net rate of photosynthesis may start to decrease as a process called 'photorespiration; kicks in, and the photosynthetic pigments become damaged by the strong light. In low light conditions where light is the rate limiting factor an increase in light intensity will cause an increase in the rate of photosynthesis - there is a limit to this however. When light intensity is high the rate may be constrained by other factors such as water or carbon dioxide availability. If we were to plot a graph of rate of photosynthesis (on the vertical axis) against light intensity (on the horizontal axis) in normal conditions (normal levels of water and carbon dioxide) there would be a steep rise to begin with but the rate of increase would start to reduce and eventually the line would be flat, if we continue into higher intensities still the line may start to drop down again.
Somehow part of my answer above was missed out: it should say that for low light intensities the rate of photosynthesis will increase with increasing light intensity. This will happen until another component (e.g. water or carbon dioxide) becomes rate limiting.