So if the radiation coming from the sun more or less covers all wavelengths of visible light, why aren’t plants black?

Emerging Sources of Energy Read more from
Chapter Energy

Let’s start with explaining the question a bit more. You can see from the graph on the preceding page that the sun provides light in almost every wavelength. The vast majority of plants, however, appear green, which means they are absorbing blue and red light and reflecting the green light back into your eyeballs. If plants were taking in all the energy the sun is providing, they would appear black, as no light would be reflected. So, to restate the question, why do plants reflect any light at all?

The easiest answer here is that absorbing red and blue energy obviously works well enough, so evolution stuck with it. Evolution, after all, doesn’t provide the best solution, but rather a solution. Nonetheless, let’s make some guesses as to why plants are green. Chlorophyll isn’t just after energy in general, but needs very specific wavelengths to pass along energy to the reactive centers of Photosystems I and II. The second reasonable guess is that too much energy is not a good thing. Absorption of light creates high-energy species, and if they aren’t used in some productive way the energy will find something else to do. These other reactions could be destructive to cells. In sum, chlorophyll only takes what it needs or what it can use.


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