Explain the experiment of two leaves to demonstrate the formation of starch during photosynthesis.

(N/A) leaf of a healthy plant is partially covered with black paper.
$(B)$ The other leaf is left fully exposed to sunlight.
$\Rightarrow$ After some time,both leaves are tested for the presence of starch using a dilute iodine solution.
$\Rightarrow$ Observation: Starch is formed in the leaf that was exposed to light (it turns blue-black with iodine).
$\Rightarrow$ The part of the leaf covered by black paper does not turn blue-black,indicating no starch formation.
$\Rightarrow$ Conclusion: This experiment demonstrates that photosynthesis,which leads to starch formation,occurs only in the presence of sunlight.