I'd suggest you read a good physics book on the properties of light, then. I really like Roger Penrose's
Road to Reality, A Complete Guide to the Laws of the Universe. It is a fairly comprehensive tome of what we know about the physics of the universe today.
But here's a very quick primer. All objects absorb light. Light comes in all different frequencies. Actually, it isn't just light, but all electromagnetic radiation of which light is just a small part. See
[URL="http
/en.wikipedia.org/wiki/Electromagnetic_spectrum"]http/en.wikipedia.org/wiki/Electromagnetic_spectrum[/URL]
But, in the range of visible light (400 nm to 700 nm), if all of those frequencies are absorbed, the object appears black to our eye. If the object only absorbs most of the spectrum, but reflects back say the 450 nm light, then the object appears blue. It is reflects back only the 650 nm light, it appears red. If it reflects back all of the visible light, it appears white.
So, this is why tannins wouldn't "condense" light or "make it stronger" because since it is black is absorbs all of the light in the visible spectrum. When light is absorbed, it is gone. It doesn't go anywhere or is kept locked away somewhere, the photons that make up light are just absorbed into the object and are gone.
This is also why I talked about that just because visible light is dampened by the tannins, that doesn't actually hurt plants too much. Plants take up light in frequencies outside of the visible light spectrum, too. I don't know exactly how much the plants take up other frequencies or how much absorption the tannins do of the other frequencies. I do know that plants have preferences of exactly what frequency light they light the best, and aquatic plants preferred frequencies aren't the same as terrestrial plants. Aquatic plants have developed different preferences because more blue light gets through water than red light (that's why water appears blue, it doesn't absorb the blue part of the visible spectrum). And, of course, it is darker underwater, so they don't need quite as high of light demands as some terrestrial plants (there are good shade plants above ground, too, that don't have high light demands).
krib, I don't want this to seem insulting, because it is not at all how I mean it, but I remember talking about light and why different objects are different colors because they reflect different frequencies of light in grade school. Didn't you cover this in your science classes you've have to date?
