Plants are green because they have a substance called chlorophyll in them. Understanding why chlorophyll is green requires a little biology, chemistry and physics.
If we shine white light on chlorophyll, its molecules will absorb certain colors of light. The light that isn’t absorbed is reflected, which is what our eyes see.
A red apple appears red because the molecule of pigment in the apple’s skin absorbs blue light, not red. Thus, we see red. Chlorophyll molecules absorb blue light and some red light. The other colors are reflected resulting in the green color that we associate with plants.
Plants get their energy to grow through a process called photosynthesis. Large numbers of chlorophyll molecules acts as the antenna that actually harvest sunlight and start to convert it in to a useful form. Here’s where the absorbent properties of the chlorophyll molecule come into play.
It turns out that eons of evolutionary design have matched the absorbance of chlorophyll to the actual color of the sunlight that reaches the leaves. Sunlight consists of primarily blue and red light mixed together, which are exactly the colors that chlorophyll molecules like to absorb. Light is a form of energy, so the chlorophyll is able to harvest the sunlight with little waste.
Resource from:
http://www.pa.msu.edu/sciencet/ask_st/081496.html
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