 When sunlight falls on a plant, you expect it to undergo photosynthesis, right and what happens when there is just a little bit of light, mostly shade? Naturally, you would expect it to do less photosynthesis and as you increase the amount of sunlight that falls on the plant, naturally the amount of photosynthesis would go up too. Let's see if we can plot all this in a graph. We are going to keep the intensity of light on the x-axis and we are going to plot the rate of photosynthesis, photosynthesis rate against the intensity of light. Now what would you expect? You would expect that when there is no intensity of light, the photosynthesis rate is zero and then as it increases, the photosynthesis rate increases, right? Like this and then you would expect as the intensity of light goes on increasing, the photosynthesis rate also goes on increasing. Turns out that that is not quite the case. So then what happens really? Well, it's true that initially when there is low intensity of light, the photosynthesis rate varies linearly with the intensity of light but as there is more and more intensity of light, what happens is the photosynthesis rate stops increasing and then just gets saturated. So the photosynthesis rate typically gets saturated at only 10% of full sunlight but why is this? Intuition would tell us that as you increase the sunlight more, the photosynthesis rate would increase more but actually there are other limiting factors like carbon dioxide and temperature. So what do I mean by limiting factors? That means they are limiting the amount of photosynthesis that can happen. Why are they limiting it? Because they themselves are not enough. For example, the temperature may be too low or too high or the carbon dioxide concentrations may be really, really low. So no matter how much you increase the amount of light, the photosynthesis rate won't increase but as long as the intensity of sunlight is low, the photosynthesis rate does increase linearly.