Let’s assume that you are conducting a popular experiment to test whether plants need light to make starch, a carbohydrate that they use to store energy: Cover part of a leaf on a living plant (geraniums are popular) with a material that will block light – black paper or tin foil work well. Expose the leaf to sunlight for several days and then remove the leaf from the plant. After removing the paper/foil, you need to break down the colored pigments in the leaf by placing it in boiling water for several minutes, until pale and soft. After boiling, cool it under tap water and pat dry with paper towel. Place the leaf in a dish and add some dilute potassium iodide solution. The iodide (which is brown) will react with starch and turn purple/blue. The places that were exposed to light (which enabled the plant to make starch) should turn blue, while the parts covered by paper will turn brown.

This experiment is described here:
experiment

Let’s examine why it works. While light, heat and starch are all forms of energy, they can’t be used in the same way. The plant needs visible light in a particular energy band to drive the photosynthetic chemical reaction. Each packet of infrared light (heat) is too low in energy to do that work, so it gets distributed into making molecules vibrate. Even if there is more total energy in heat, the molecular vibration is too difficult to concentrate in one place to drive the photosynthetic reaction. Thus, only the parts of the leaf that have access to sunlight can easily build starch. Starch is a way of storing the energy so that it can be moved around the plant and broken down to power other chemical reactions that allow the plant to live and grow.

Where you block access to sunlight, the leaf can’t make more starch but that doesn’t mean the cells there stop working. Instead, they keep carrying on chemical reactions using the starch they made and stored previously, eventually consuming it. That’s why these areas stay brown after you dye the leaf with iodine – there isn’t enough starch for the iodine to react with.

Now, do you have to use black paper or tin foil? No, anything that significantly blocks the useful wavelengths of light will give you similar results. Try using white paper – it will reflect most of the light away from the leaf too. Or, try colored transparent plastic. The color the plastic appears is the opposite color that it absorbs, e.g. you can block red light by using green plastic. How could you determine which wavelengths are the most useful for photosynthesis? Stay curious!