Aim : To prove that light is essential for photosynthesis.
Apparatus : A potted plant, a light screen, beaker, iodine solution.
Procedure : Keep a potted plant in darkness for two days to make us sure that the leaves are free from starch. Attach a light screen to one of the leaves. The light screen holds the leaf by a lid. The lid has a cut design like a star water the plant and keep it in sunlight for 4 or 5 hours keep in mind that the leaf with light screen receives sunlight on the leaf through the cut design in the lid.
Then detach the leaf from the plant and remove light screen. Test the leaf for starch using iodine solution.
Observation : You will see blue colour only in the area which received the sunlight. This area resembles the cut design on the lid of the light screen. Rest of leaf is colourless.
Inference : In blue coloured portion only carbohydrates are present. That means the part of the leaf which received sunlight performed photosynthesis. Rest of the leaf remained colourless due to non-performance of photosynthesis. 

Conclusion : From the above experiment, it is clear that light is essential for photosynthesis.
Precautions : The potted plant should be kept in darkness for two days or three days.