Question

To show experimentally that light is necessary for photosynthesis

Answer 1

for instance , why plants perform photosynthesis in day not in night ?
because in day plants get huge amount of sunlight but in night there is no source of energy. So light is necessary for photosynthesis.

Answer 2

Procedure

Detach the leaves of a potted plant.

Cut a simple shape from a piece of thick cardboard of aluminum foil and fix it onto the upper and lower surface of some of the leaves using paper clips, such that part of the leaf is covered whilst other parts are not covered.

Put the plant in sunlight for three (3) hours.

Remove the cover and test the leaf for the presence of starch.

Observation

The areas covered by the thick cardboard or aluminium foil tested negative for the the presence of starch but the area exposed to the sunlight tested positive for the presence of starch.

The presence of starch in the uncovered portion and it's absence in the covered portion shows that sunlight is necessary for the formation of starch.

As starch is formed by photosynthesis, sunlight must be necessary do photosynthesis.