Take a potted plant having green leaves and place it in a completely dark place for about three days to destarch its leaves. Take a thin strip of aluminium foil (or black paper) and wrap it in the centre of one leaf on both the sides. The aluminium foil should be fixed tightly to the leaf by using paper clips so that sunlight may not enter it from the sides. The aluminium foil should cover only a small part of the leaf so that the remaining part of the leaf remains uncovered and exposed to sunlight. Keep this potted plant (with partially covered leaf) in bright sunshine for three to four days.
Pluck the partially covered leaf from the plant and remove its aluminium foil. Immerse this leaf in boiling water for a few minutes. This will break down the cell membranes of leaf cells and make the leaf more permeable to iodine solutio. This leaf is now to be tested for the presence of starch. But before testing for starch, chlorophyll has to be removed from the leaf. This is because chlorophyll interferes in the test for starch due to its green colour. Put the plucked leaf in a beaker containing some alcohol. Place the beaker containing alcohol and leaf in a water bath. The boiling alcohol will remove chlorophyll from the green leaf. Boil the green leaf in alcohol till all its green pigment chlorophyll is removed. The leaf will become almost colourless or pale (and the alcohol will turn green). Remove the colourless leaf from alcohol and wash it thoroughly with hot water to soften it and remove any chlorophyll which may be sticking to it.
Place the colourless leaf in a petri-dish. Drop iodine solution over the decolourised leaf with the help of a dropper. Observe the change in colour of leaf.
The middle part of leaf which was covered with aluminium foil does not turn blue-black on adding iodine solution showing that no starch is present in this middle part of the leaf. This is because sunlight could not reach the covered middle part of the leaf due to which this covered part of leaf could not carry out photosynthesis to make starch. The uncovered part of leaf (on both sides of the aluminium foil) which was exposed to sunlight turns blue-black on adding iodine solution showing that starch is present in this part of leaf. This means that the part of leaf which was exposed to sunlight carried out photosynthesis to make starch.
This shows that sunlight is necessary for photosynthesis.