Ok, I wrote this description, then realized you said "black & white" paper, but I've heard of a pin-hole project that literally just used black paper, and the image is formed by fading. So that's where all this came from.....
Originally Posted by danube
.....This is a very old phenomenon, noticed early in the history of photography. It has some analogues to Uto paper, but I'm not qualified to draw any at the moment (analogies that is). Here's my best crack at explaining this....
Blue light from the sky hitting the paper will appear blue, right? Therefore, that paper is reflecting blue and absorbing every other color of light. By absorbing everything else, the paper is taking in that energy, not bouncing it back, so it has a damaging/fading effect. I'm sure when you get down to the physics, it's bumping electrons off or something to that effect (I really have no idea).
So the colored light is destroying the elements of the dye in the paper that don't reflect blue light, and after a long enough time, you are left with a color.
Now, I'm struggling to make sense of this myself, but I've read about it and this should put your head in the right ball park to understand it. I've got some books that have great explanations of this... I'll do some searching on Google books in the meantime.
update: ok, here it is from J.S. Friedman's 'History of Color Photography'...
A yellow dye appears such because the ultraviolet and blue rays are absorbed, and the green and red rays are trasmitted unaffected. A yellow dye will fade, therefore, only when it is exposed to blue or UV light. A magenta dye will absorb the green rays, hence its sensitivity will be limited to the green range. A cyan dye will absorb the red rays, so its sensitivity will be confined to that region. If a yellow, cyan and magenta dye be mixed in equivalent portions, the yellow will absorb the blue, the magenta will absorb the green and the cyan the red. Together, they will absorb all the visible light. If the dyes are fugitive and bleach upon exposure, a very interesting phenomenon results. Suppose we expose the mixture to blue light. This will affect only the yellow dye, since only that will absorb blue (we assume ideal dyes), and thus only the yellow dye will fade or bleach, dependent upon the length and intensity of exposure. If all the yellow dye be bleached, there will remain an equivalent mixture of the cyan and magenta dyes. The cyan will absorb the red primary and transmit the blue and green. The magenta will absorb the green primary and transmit the other two. Only the blue primary will be transmitted by both acting simultaneously. Thus an exposure to blue will yield blue. A similar analysis will show that, regardless of what the color of the exposing light is, the resultant image will be as close an approach to it as is possible under a 3-color subtractive system utilizing these dyes as secondaries.
We have to assume that the black paper used has some mixture of dyes in it.
Last edited by holmburgers; 10-14-2010 at 03:09 PM. Click to view previous post history.