Photosynthesis is the ability of plants to absorb the energy of light, and convert it into energy for the plant. To do this, plants have pigment molecules which absorb the energy of light very well. The pigment responsible for most light-harvesting by plants is chlorophyll, a green pigment. The green color indicates that it is absorbing all the non-green light-- the blues (~425-450 nm), the reds and yellows (600-700 nm).

Red and yellow light is longer wavelength, lower energy light, while the blue light is higher energy. In between the two is green light (~500-550 nm).

It seems strange that plants would harvest the lower energy red light instead of the higher energy green light, unless you consider that, like all life, plants first evolved in the ocean. *Sea water quickly absorbs the high-energy blue and green light, so that only the lower energy, longer wavelength red light can penetrate into the ocean. Since early plants and still most plant-life today, lived in the ocean, optimizing their pigments to absorb the reds and yellows that were present in ocean water was most effective. While the ability to capture the highest energy blue light was retained,the inability to harvest green light appears to be a consequence of the need to be able to absorb the lower energy of red light.

Plants also use multiple variants of chlorophyll, as well as accessory pigments such as carotenoids (which give carrots their orange color) to tune themselves to absorbing different wavelengths of light. That makes it impossible to assign a single wavelength of best absorption for all plants. All plants, however, has chlorophyll a, which absorbs most strongly at ~450 nm, or a bright blue color. This wavelength is strong in natural sunlight, and somewhat present in incandescent lights, but is very weak in traditional fluorescent lights.

Special plant lights increase the amount of light of this wavelength that they produce. But a 400-500 nm wavelength bulb wouldn't be enough, since many plants take cues for germination, flowering, and growth from the presence of red light as well. Good plant lights produce red light as well, giving plants all the wavelengths of light they need for proper growth.

Note from ScienceLine Moderator

Below is a correction to a paragraph on this answer sent to the ScienceLine site on June 12th, 2013:
Paragraph"Sea water quickly absorbs the high-energy blue and green light, so that only the lower energy, longer wavelength red light can penetrate into the ocean. Since early plants and still most plant-life today, lived in the ocean, optimizing their pigments to absorb the reds and yellows that were present in ocean water was most effective.

*Correction: In fact, red light gets absorbed by the ocean most quickly, and it is only blue light which can penetrate to any depth.

In reference to the answer above:

"It seems strange that plants would harvest the lower energy red light instead of the higher energy green light, unless you consider that, like all life, plants first evolved in the ocean. Sea water quickly absorbs the high-energy blue and green light, so that only the lower energy, longer wavelength red light can penetrate into the ocean."

The exact opposite actually is true, blue light with the short wavelength colors such as blue actually penetrating the deepest, while long wavelength colors such red light penetrates the shallowest. Not to mention that in coastal waters green light is actually the spectrum that penetrates deepest....

oceanexplorer

H₂O liquid must have some red and infra red absorption bands. Not to mention the particulate debris in water

Since the particles' sizes are of much bigger wavelength than EM, the red wavelengths will be more scattered than the blue, so that also will not favor transmission of the redish photons.