Answer/Explanation:
Set up 4 highly similar (same species, ideally same size and health) water plants under exactly the same conditions except for light intensity.
Cover one plant with a cover such that no light can get through
Cover the other plants such that no light can get through except for the light from the following lamps:
Put one plant under a semi-dull lamp
Put one plant under a bright lamp.
Put one plant under a very bright lamp
This means you have entirely controlled how much light gets to each plant, but everything else is the same