The simplest way to observe the effects of photosynthesis is to see what happens when an essential element, sunlight, is removed. Plants use sunlight to make food, so without it the plant will ultimately die. In a basic experiment, take two of the same plant and place one by the sunlight and one in a closet for a set number of days. Be sure to provide both with an equal amount of water to ensure that sunlight is the only thing affecting your results. After a set amount of time, observe that the plant in the closet does not grow as well, and the leaves lose their vibrant green color.
Another way to approach a photosynthesis science project is to vary the color of the light and study the results on plant growth and coloration. Light comes in a spectrum of different wavelengths. When plants absorb sunlight, they are receiving a full spectrum of all colors. Limiting light absorption to one color, meaning one specific wavelength, will likely affect the plant. Do this with several colors, and compare it to a control plant that receives normal sunlight.
Autumn is the best time to do a science project about photosynthesis and plant pigments. As winter approaches, sunlight becomes more scarce and many plants are less able to produce food energy. This is why leaves turn different colors in the fall. A simple experiment is to find some leaves on a tree during summer that is known to turn color in the fall, and pinpoint which leaves receive bright sunlight. Cover part of these leaves with aluminum foil. When the rest of the leaves on the tree turn bright red or purple, remove the foil and observe that the covered area is still the same color it was in the summer.
Paper chromatography is a useful and more precise way of studying the effects of photosynthesis on plant pigments. Chromatography is a scientific procedure that separates mixtures; it is especially useful for separating pigments. The basic procedure begins by preparing a plant extraction; mash leaves in a tall jar with a solvent such as rubbing alcohol and strain out the plant bits. Let the mixture sit in the dark for about 30 minutes. Then take a strip of paper towel and place one end into the mixture while allowing the other end to drape over the jar edge. As the liquids travel up the paper towel, different strips of color will appear, and the different pigments will be visible. It is preferable to do this with leaves at all different stages of coloration.