A student investigated the effect of light intensity on the rate of photosynthesis of pondweed. The light source was placed at different distances, ddd, from the pondweed. The relationship between light intensity, III, and distance from the light source, ddd, is described by the inverse square law:
I∝1d2 I \propto \frac{1}{d^2} I∝d21Calculate the relative light intensity when the distance d=20 cmd = 20\text{ cm}d=20 cm. Give your answer in standard form with the unit cm−2\text{cm}^{-2}cm−2.
When the distance d d\,d was decreased from 10 cm to 5 cm, the light intensity increased significantly, but the rate of photosynthesis remained constant. Explain why the rate of photosynthesis did not increase when the light intensity was increased in this range.