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How do we measure light?
Visible light is part of the larger electromagnetic scale which includes invisible spectrums such as radio waves and x rays. Each spectrum represents an electromagnetic frequency measured in nanometers (one billionth of a meter):
Do plants use all light spectrums produced by the sun?
Most indoor growers seem to believe that the best indoor grow lights would have the same light spectrum as the sun – a relatively full spectrum over the visible light frequencies. After all, plants evolved over millions of years to best convert light energy into carbohydrates and sugars. The most readily available light from the sun is in the middle part of the spectrum which we see as green, yellow and orange. These are the primary frequencies that human eyes use. However, studies show that these are the least used light frequencies in plants. Most of the photosynthetic activity is in the blue and red frequencies.
The main reason for this counter-intuitive use of light by plants seems to be related to early forms of bacteria and the evolution of photosynthesis. Photosynthesis first evolved in bacteria over millions of years in the primordial sea. This evolved in bacteria long before the appearance of more complex leafy plants. These early photosynthetic bacteria extensively used the yellow, green and orange middle spectrums for photosynthesis which tended to filter out these light spectrums for plants evolving at lower levels in the ocean. As more complex plants evolved at lower levels they were left with only the non-filtered spectrums not used by bacteria – mostly in the red and blue frequencies. The yellow, green and orange light is mostly reflected off the surface of the leaves and this is why photosynthesizing plants are green
Do different light spectrums do different work in plants?
Not only do plants focus on specific light spectrums for photosynthesis but different light spectrums are used for different types of growth in plants. There are millions of photosynthetic receptors in a leaf of a green plant. Each receptor includes specialized pigments that absorb specific frequencies during photosynthesis. By measuring the amount of oxygen produced under various light spectrums we can measure the amount of photosynthetic activity under each light spectrum. This has produced a very detailed map of which light spectrum is related to which type of plant growth.